Most travelers are aware that condominium apartments can be rented in major beach, mountain and ski resorts throughout the United States. The difficulty, however, often has been finding out exactly what places are available and when.

Most travel guides are of little help, providing primarily the names of hotels and motels. A resort community's chamber of commerce can provide a list of condo managers, but then you must make the phone calls. It's hardly a convenient process, especially if you are not familiar with the resort you want to visit.

With the growing popularity of condo holidays, however, a new type of service has emerged in recent years to help vacationers find condominium apartments quickly and easily.

There now are several national condo-rental clearinghouses, each offering a selection of thousands of condominiums at resorts across the United States and, increasingly, in Europe and the Caribbean.

With a single, usually toll-free call, you can get information about -- and make reservations for -- a condominium apartment in such top-rated resort areas as Hawaii, California, Florida (the beaches and Disney World), New England and the Rockies (winter or summer sports).

The idea behind the condo networks, which do not charge for the service, is to make renting a condo as convenient as phoning the central reservation number of a national hotel or motel chain. Travelers can reserve an apartment on their own or through a travel agent.

Condo apartments are a vacation alternative to hotels and motels. They provide certain amenities, such as kitchens, living rooms and laundry facilities, that usually are not available in hotels and motels. But they also lack many of the services hotels and motels offer.

In resort communities, you are as likely to find condos with a beach front as hotel and motels, and many condos also have private swimming pools, whirlpool tubs, saunas, tennis courts and other sporting facilities.

On the other hand, most do not provide daily maid service, and some offer no maid service at all. And, unlike hotels and motels, there is probably no restaurant on the premises, no magazine rack or drugstore counter in the lobby and certainly no bellhops to assist with the luggage. Some condos also don't take credit cards.

To many vacationers, however, these are extras they can do without. They are looking for the home-like atmosphere that can be found in condominiums.

Condos have proved especially popular with families. The kitchen is a major reason, since it enables parents to save money by preparing some of the family's meals, even if only breakfast. Individuals can eat in pajamas (or bathing suits) at whatever hour they wake up.

Usually there's more room in a condo for a family to spread out -- an important consideration when a vacation stretches a week or more. And, say clearinghouse operators, a two-bedroom, two-bath apartment often can be rented at a price not much higher than one room in a hotel or motel.

But families aren't the only guests attracted to condos. Two or more couples vacationing together can share the cost of a two- or three-bedroom place and enjoy the convenience of a kitchen and sitting room for card games or a private party.

One more advantage to condos, say clearinghouse operators, is that frequently vacancies are available during a resort's busy season when every hotel and motel is booked solid.

Condominium apartments range in size from studios to one-, two- and three-bedroom places and in quality from comfortable to luxurious. The price, from about $200 to $1,000 or more a week, climbs in the resort's high season and drops in the low season.

Most apartments are owned by individuals or investors, either outright or for a specified week or two as part of a time-share program. The owners make the apartments available for rent when they are not using them. Customarily, a local management firm maintains the apartments and facilities, and some provide a rental service.

The new national clearinghouses have linked the various condo apartments, buildings and complexes with the single toll-free number. You tell the reservation clerk where you want to go and a price you hope to pay, and generally you will be offered several choices.

Each condo management differs, so policies differ on procedures for reservations, deposits, method of payment, maid service and minimum length of stay. Make sure the reservation clerk explains the policy applying to the condo you want.

Among the clearinghouses:

*Vacation Resorts International: Organized in 1983, the Atlanta-based firm represents more than 75,000 privately owned condominium units, principally in the beach resort communities of Florida, as well as Georgia, the Carolinas, Hawaii and the Caribbean.

It also has a large listing of ski resorts, including Aspen, Colo. and Stratton, Vt., available year-round.

The firm publishes a quarterly color magazine, "CondoCations," which provides a description, rental prices and photos of each of its warm-weather condo facilities. A copy sells for $3. Two supplementary guides will be available soon, one for ski and mountain resorts and the other for golf and tennis resorts.

The magazine and guides make it possible for vacationers to pick a condo that appeals to them before phoning for a reservation, says John Hunt, sales vice president.

So far, the beach condos represented all are beach-front properties, he says, but the firm plans soon to include less-expensive condos a short walk from the beach.

For more information: Vacation Resorts International, 5505 Interstate North Parkway, Suite 150, Atlanta, Ga. 30328-3074, (404) 956-2990 or (800) VRI-0009.

*Holiday Condominiums: One of the oldest and largest of the national condo clearinghouses, this firm -- begun in 1980 -- represents 200,000 resort apartments in the United States, Europe and the Caribbean. They include warm-weather and ski destinations.

On its list are more than 100 resorts in North America designed with special facilities for disabled travelers.

Founder Elliott Hechtman says his firm frequently can offer condos at discounted rates because the owners are anxious to rent them, to help meet the mortgage. A current offer is the "no-frills vacation" at $349 for a week's stay in a studio, one- or two-bedroom apartment (depending on what's available) at 50 resorts.

The growth of his business has been astonishing, from a minimal staff in the beginning to 75 employes presently.

For more information: Holiday Condominiums, 7701 Pacific St., Suite 300, Omaha, Neb. 68114, (402) 392-0468 or (800) 228-0002.

*Reservations International Inc.: Since its founding in January 1984, this Arkansas-based firm has concentrated on warm-weather beach resorts from the Carolinas south to the Florida Keys and west to South Padre Island, Texas, and Mexico's Caribbean coast. But it has a share of ski resorts, too, principally in the major resorts of Colorado but also nearby Canaan Valley in West Virginia.

In all, it represents about 3,500 apartments, some of them "ultra-luxury," as President Hal Vanatta puts it, in 100 separate condominium complexes. He got into the clearinghouse business, he says, when he attended a Harvard University business seminar where he met condo executives looking for a way to market rentals and other executives who enjoy condo accommodations but had trouble finding what places were available.

Vanatta's clearinghouse also publishes a regularly updated guide to its properties, distributed mostly to travel agencies but also available to individuals for $11.50. The firm does the bulk of its business through travel agents, who, says Vanatta, also can help a vacationer arrange for air line tickets and auto rentals, if needed.

For more information: Reservations International Inc., Box 5, Desoto Center, Hot Springs Village, Ark. 71909, (800) 643-1000.