"It's probably the fastest-growing segment of the tourist industry today," declares Jim Host, spokesman for the National Tour Brokers Association.

Let's see. Package deals combining discount air fares and hotels? Caribbean cruises? Camping in national parks? Amtrak? What's left?

Bus charters, that's what. The unsung mainstays of the bus industry are rolling to new heights.

Charters carried 55 percent of the 335 million intercity passengers who used the nation's 1,000 companies last year. That marked the first rise in total ridership since 1974. The charter share is up from 45 percent in 1977, when there were 332 million riders.

While the charter business is on the rise, total ridership has dropped by 16 percent from the 401 million passengers in 1970. With spot fuel shortages, weekend service station closings and the unrelenting rise in gasoline prices, the charter bus tour business should be booming. The business is, but the latest energy crisis is not the reason. In fact, it may be hurting charters.

Greyhound spokesman Lee Whitehead explains that regularly-scheduled business has become so heavy in recent weeks that as many as 50 percent more buses are needed to accommodate riders on some routes. The industry giant has 4,200 buses.

"Buses that normally would be in charter service are now required in regular route services," Whitehead says. "Therefore, charters are becoming more difficult to come by."

The fears associated with fuel availability and cost are sparking requests for lengthy contracts from real-estate groups that specialize in development of leisure properties and retirement homes.

"Some of the groups contacting us for long-term agreements are thinking that even if the shortage eases off like it did after [the 1973-74 Arab oil embargo], maybe we should do something," said Lee Sneath, Trailways' spokesman.

For the past several years, companies ranging in size from Greyhound and Trailways down to one-bus lines have complained they were speeding down the road to financial ruin. The federal government was to blame, they maintained, for ignoring intercity buses when formulating energy conservation strategies and for lavishing more than a half-billion dollars a year on Amtrak, the bus lines' favorite whipping boy.

If a bus charter appeals to you, your newspaper travel section is a good place to start investigating. Travel agents and tour brokers may be advertising trips to a place you've always wanted to visit. Travel agents also may know of other charters or find a way to combine some other form of transportation with a charter or tour at the destination.

If riding with strangers for several days isn't appealing, get you own group together. Most intercity buses seat 44 to 47 people, and companies can be found in the Yellow Pages. Check with people who have taken bus tours or used the company.