Even some older Americans jump to the wrong conclusions about tours designed for travelers in the over-50 and over-60 age brackets. The pace, they say, might be a little too slow, and they don't want to spend their vacation napping on a bus.

Well, they needn't worry. Senior travelers, it seems, are a rather hardy lot.

"They are very active. They do not want to sit on a beach and bake. They want to be on the go. They want to see it all. They aren't going to miss a thing," says Marie Shaffer, marketing manager for Saga International Holidays, one of a number of travel firms catering to the senior traveler.

The older population makes up a growing segment of the nation's population -- there are 50 million Americans over 55 -- and the vacation travel industry sees them as a lucrative market. Many have the time, the health, the money and the inclination to travel, and they are doing so into their eighties and nineties.

Senior tours range from cruises to the Grand Tour of Europe and excursions throughout the United States and the rest of the world. One of the most popular trends is the opening of apartment-hotel complexes for older travelers in U.S. and European resort areas.

"Generally speaking, senior citizen travelers have traveled more. They are more apt to be looking for their second-, third-, fourth-, fifth- or sixth-time destination than the younger traveler," says Patricia Lyons, marketing vice president of Grand Circle Travel, which also specializes in senior-citizen tours.

They are adventurous, too. "Surprisingly," she says, "in some ways more than younger people. They've reached an age when they're not caught up in what they should do. They do what they want to do. They think, 'This might be my only chance, so I'm going to do it.' "

"There's not a place in the world the older traveler won't go," adds Bob Dunn, travel service vice president of the American Association of Retired Persons, which has an extensive catalogue of tours that include South America, the South Pacific and Africa.

As much as anything, say tour operators, older travelers are drawn to senior tours by the opportunities to meet other people of their age. So many friendships have been formed on Saga's tours, says Shaffer, that the firm has established a "club" to help participants keep in touch.

"It's a sharing of experience," says Dunn, "these 25 or 30 like-minded souls on the same trip." Travelers will tell him, "I may not remember the Taj Majal, but I'll always remember Sadie."

Certainly there are many independent seniors who get about the country and the world quite well on their own, and they might find organized travel confining. But group travel can be less expensive, and many travelers enjoy the security of having a tour escort available to resolve problems, especially in a foreign country.

How do tours and other vacation trips for older age groups differ from tours available to the general public?

Only in subtle ways, actually, say tour operators. The destinations tend to be the same, although older travelers generally are more interested in sightseeing opportunities than relaxing on a beach. One theory is that they do their relaxing at home and so are primed for a more active holiday.

Among the differences:

Saga emphasizes longer tours because, says Shaffer, "our customers tell us they don't want to be rushed. They hate to have to pack and unpack every night." This means stopovers of two or three nights instead of just one. En route to Kenya for a camera safari, for example, the firm schedules two nights for rest in a London hotel after the transatlantic flight.

Grand Circle Travel chooses hotels with a minimum of steps, which might trip up some of its customers, and rooms with a lot of light. The firm's buses come equipped with entrance steps that reach to the pavement. "We just look for small things, which in some ways we don't like to point out, because we don't want the travelers to feel old," says Lyons.

Several firms, including Grand Circle, offer stays of two weeks or more in inexpensive apartment hotels for seniors in major tourist destinations, such as the French Riviera and Spain's Costa del Sol. Staff members are available for any assistance needed. "It's a chance to live as a native -- with our representative at hand," says William J. Alderson of Passages Unlimited, a new entry into the senior traveler's market. Some travelers stay on for a season or for a year.

The American Association of Retired Persons seeks out centrally located hotels. "Older people are not terribly keen about a view of the beach," says Dunn. Instead, they usually prefer to be "in the midst of everything," where they can window-shop, stroll the town and go to museums.

Frequently, the tours are less expensive, because older travelers, with no job or child-raising obligations, often can take advantage of off-season rates. In the apartment-hotel programs, says Grand Circle's Lyons, they often can stay more cheaply abroad than they can at home.

"People over 60, regardless of their financial situation, are really eager to be sure they are getting the best value," says Saga's Shaffer. "They will do a lot of comparison shopping," in part because they have the time to search out the best buys. "They keep us on our toes."

Here are some of the major operators of senior tours (one traveler in the party is expected to meet the minimum age requirement):

Grand Circle Travel: In the senior travel business for 26 years, Grand Circle operates a number of apartment hotels in the United States and Europe, dubbed the "Extended Vacations" program for travelers over 50.

For a minimum two-week stay in the Spanish resort town of Torremolinos, the price ranges from $685 to $725 per person for a studio apartment, including round-trip air fare from New York, transfer to the hotel, maid service and a program of cultural and social activities. Additional weeks are only $50 to $55 per person, or less than $15 a day for a couple. Free lectures, U.S. movies and other social activities are provided several nights a week.

Cruises and motorcoach tours are also available.

For more information: 555 Madison Ave., New York, N.Y. 10022, (800) 847-4240.

Saga International Holidays: This is a 33-year-old British travel firm that opened its doors in the United States four years ago. This means Americans probably will find travelers from Great Britain, Australia and other English-speaking nations on the same tour. Saga caters to over-sixties with motorcoach tours and cruises throughout the world.

For older adventurers, Saga offers a 21-day "Kenya Safari" that includes several nights in game park lodges for $2,199 per person, including air fare from Washington, lodging and most meals. A 14-day bus tour of the national parks of the Rocky Mountains is $1,339 per person, including air fare to Denver, lodging and two meals daily.

For more information: 120 Boylston St., Boston, Mass. 02116, (800) 343-0273.

Passages Unlimited: For over-fifty-fives, this new firm features apartment living in London and Paris and such resorts as Antibes on the French Riviera and the Greek island of Corfu. Also offered are standard motorcoach tours of Europe and "special sojourns" to Greece, India and the Soviet Union. Allied with Global of London, a major British tour operator, the firm also mixes English-speaking nationalities.

A two-week stay in a Paris apartment (through Aug. 13) is $1,199 per person, and includes round-trip air fare from New York, 14 nights in a modern apartment, weekly maid service, a one-week Paris Metro pass and medical and travel insurance. Extra weeks are $179 per person.

For more information: 48 Union St., Stamford, Conn. 06906, (800) 472-7724, or see a travel agent.

* Travel Service, American Association of Retired Persons: The Travel Service's 200 tours and cruises, throughout the United States and the world, are open to over-fifties who are members of the American Association of Retired Persons ($5 to join). An 18-day escorted tour of China this summer is $2,895 per person, including lodging, meals and air fare from San Francisco. A 19-day escorted tour of Egypt and Israel, including a six-day Nile River cruise, begins at $2,959 per person for lodging, meals and air fare from New York.

The organization also has begun offering apartment-hotel vacations in cities and resorts, which it calls its "Leisure Holidays" program. A modern apartment in Rome from May through September is $230 per person (double occupancy) the first week and $155 for additional weeks. This includes airport transfers, baggage handling and an "initial supply of basic groceries." Air fare is $679 to $729 per person from New York.

For more information: 5855 Green Valley Circle, Suite 300, Culver City, Calif. 90230, (800) 227-7885.

* Yugotours: Yugoslavian-owned, this tour operator not surprisingly features trips (for over-sixties) to Yugoslavia, principally two-week stays in "first-class" hotels at Adriatic Sea resorts in fall, winter and spring. Destinations include the ancient fortress city of Dubrovnik, the island of Hvar and the Old World resorts of Opatija and Hercegnovi. Short excursions to Rome, Athens and Istanbul and extended tours of Egypt, Greece and Russia are also available. Through May 9, a two-week stay in Dubrovnik is $919 per person, including air fare from New York, 13 nights lodging, two meals daily, airport transfers and hotel taxes and gratuities.

A similar summer program, from May 16 to Oct. 3, includes two-week stays in either Dubrovnik, Opatija, the coastal town of Budva or the inland lake resorts of Bled and Ohrid. The 13-night price for Dubrovnik (the price varies slightly for each destination) is $999 to $1,099 per person, including air fare and two meals daily. Each additional week is $224, or slightly more to split the holiday between two of these destinations.

For more information: 350 Fifth Ave., New York, N.Y. 10118, (800) 223-5298, or see a travel agent.

* Gramercy Travel System: For a long time, this 28-year-old tour operator was known for its "Single World" program. In December, it added "Senior World." So far, the new program concentrates on cruises for the over-45 age bracket (though most participants are over 60). Because it books cabins in bulk, the firm says it can offer week-long Caribbean cruises from $400 to $500 per person less than regular quoted fares.

Initially, Gramercy organized tours aboard the Norway, the Norwegian Caribbean Lines' huge luxury liner sailing to St. Thomas in the Virgin Islands and Nassau in the Bahamas. But it has expanded to include European cruises aboard Royal Viking Line ships because, says cruise manager Wendy Lowenstein, older passengers generally are more interested in sightseeing in ports than nourishing their tans under a tropic sun.

Cruise passengers are guaranteed lower berths, and Gramercy sends along its own tour escort and arranges its own shore excursions.

For more information: 444 Madison Ave., New York, N.Y. 10022, (800) 223-6490, or see a travel agent.