Hold the bat firmly -- and keep your ear on the ball!

Before he retired, Stuart Hale headed a group of engineers working on magnetic materials at Bell Laboratories. Now he drills holes in baseballs.

But that's all right. After he's finished, Julia Mazalenski, a retired coil winder at Western Electric, takes over, restitching the balls with a long curved needle.

In between, some 20 other retirees - all members of Telephone Pioneers of America, the phone company's 550,000-member nationwide service organization - get involved, wiring and soldering a solid-state electronic module to stuff into the large, kapok-filled softballs.

The result: a beep baseball. On the outside, it looks like any other softball - except for a star-shaped pattern of tiny holes. Pull out a thin stem, however, and the ball starts beeping loudly through a tiny speaker behind the holes.

Who wants beeping baseballs? The National Beep Baseball Association, of course - a group of blind athletes whose seventh national tournament opens Aug. 26 in Minneapolis. ''We estimate that there are over 200 teams in the US,'' says association spokesman John Ross. This year, 15 teams are gathering - including last year's national champion from Albuquerque, N.M.

Baseball for the blind developed from an idea hatched by Charles Fairbanks, a Mountain Bell engineer in Colorado, in 1964. Involved in community service projects himself, he learned that blind children could readily play catch if the ball made a noise.

So he designed a sound module inside a throw pillow. From there he went to baseballs, and the game began. As now played, the ball is thrown by a sighted pitcher to a blind batter. He or she hits it and runs to a base - a highway cone containing a beeper with a different sound. If the batter reaches base before the fielders (who are also blind) find the ball, he scores a run.

Telephone Pioneers officials estimate that, in addition to the organized teams, as many as 1,000 blind youngsters play the game. Back in 1970, there were only some 70 beeping baseballs in existence. Artie McGrath, a former Pioneers coordinator, recalls that ''the first audio ball we saw cost $40 and took over 40 hours to assemble.''

But telephone company engineers went to work on the design - using, for example, the same tiny speakers used in the Princess phone. The Pioneers in North Andover, adopting ''Audio Ball'' as a service project, now make about 30 balls every Tuesday morning in a basement room of the Western Electric plant here. With volunteer labor, and with space, tooling, packaging, and design provided by Western Electric and Bell Laboratories, the balls (complete with battery recharger) cost about $25 apiece.

Since they began in 1970, says Pioneers coordinator Tippy O'Connell, the retirees have produced more than 7,000 balls. They sell them to other Pioneers chapters, which then donate them to organizations that work with the blind. And because the balls are made for the blind, the post office ships them to their destination for free.

So far, the North Andover chapter is the only source of supply, although the Minneapolis chapter rebuilds balls that have lost their beeps. ''They've gone all over the world,'' says O'Connell, mentioning orders from Israel, Greece, Ireland, and Italy. No one is quite sure how long they are used (''They last from one day to 10 years,'' says Mr. McGrath), but the demand increases.

If the players love it, so do these retirees. Julie Levesque says, ''I look forward to it every Tuesday morning,'' - especially because there is ''no clock punching.''

It must be a very interesting game to watch. ''It must be,'' agrees O'Connell , who is released from his job at Western Electric every Tuesday morning to coordinate the volunteers. Then he adds, ''We've never seen it played.''

But do they get letters? ''Oh yes,'' says O'Connell, ''we get 'em in Braille.''