The ubiquitous popularity of such portable tape players as the Sony Walkman offers an opportunity that entrepreneurs are beginning to take advantage of.

Jill Fallon, for instance, formed Boston Walkabouts, a division of The November Co., in the spring of 1983 with a $50,000 bank loan and the notion that the portable players could be used to provide walking tours of the city. Her guide to Boston's historic Freedom Trail is available in four languages at gift stores, hotels, and tourist stops throughout the city for $9.95 a tape.So far, she has sold about 4,000 of them, and she has some large-volume sales to conventions pending.

Fallon is by no means alone in the taped-tour gusiness. Similar guides are already available in New York City, New Orleans, Philadelphia, Washington, D.C., Charleston, S.C., and more than 70 other tourist destinations, most produced by small, usually local companies.

Over the years there has been a small market for tape-recorded tours of some cities and state parks that have been available for rent on an Acoustiguide -- the somewhat unwieldy, battery-run tape machines used most popularly to provide museum-goers with audio gallery tours. But the convenience of portable tape players has created a new market, and Fallon predicts that tape-recorded guides will becomes as standard as Fodor's guidebooks.

"The challenge lies in distributing [the tape], in letting people know it's available, it's out there," explains Heidi Dietschi-Cooper, whose company, Head Trips of South Strafford, Vt., began offering a tape-guided tour of Montreal this past spring.

Dietschi-Cooper designed her tape to "stack up with books and sell like one," packaging it in a white box with writing along the "binder." Annabelle Simon-Cahn, yet another entrepreneur, sells Travelcassetts/Artours, taped tours of cities with a particular eye on art and architecture, and markets her products primarily through direct mail. Recently, she compiled a catalog that lists more than 80 walking- and driving-tour tapes available to consumers from Artours and other companies. Mailing the catalog to an established client base and peddling it at this year's American Booksellers Association Convention, Simon-Cahn had her first good selling season last year.

The best market for tape tours are tourists who are already in the destination city or state. Selling to them, however, has its own problems.

"Small recorders eventually will be as natural a part of a traveler's baggage as the alarm clock and pocket calculator have become," predicts Fallon. "But not yet. Frequently people don't think to travel to a new city with their [tape player]." Anticipating this possibility, Fallon provides small tape players for rent at hotels and tourist centers throughout the city.

But if tourists do get into the habit of bringing their tape players along when they travel, they will make a ripe market for yet another cassette-tape product. Washington, D.C.-based Capitol Hill Hospital has introduced an audio program of isometric exercises called "Fitness in Flight," designed to take the edge off jet lag.