It surely never occurs to Tarzan as he swings his way through the jungle issuing blood-curdling yodels that he should put a trademark on those primal shouts to discourage imitators. But we civilized people have improved on his primitive ways.

Last month, two companies registered the ape man's confident cry as a trademark with the U.S. Customs Service. That means customs officers at ports nationwide will now be watching - or, to be precise, listening - for any counterfeit jungle-man dolls that make Tarzan sounds.''This is the first time anyone has recorded a sound trademark with Customs,'' said David Ehrlich, a trademark attorney with the law firm of Fross, Zelnick, Lehrman & Zissu, New York. It also appears to be the first sound trademark for a toy, he added.

Ehrlich represents Edgar Rice Burroughs Inc., of Tarzana, Calif., the successor to Edgar Rice Burroughs, who created the Tarzan character in 1912. It licensed the Walt Disney Co. to make the summer blockbuster movie ''Tarzan.'' The two companies then registered the trademark with the U.S. Patent and Trademark Office and with Customs.

Counterfeit Tarzan figures occasionally make it to flea markets, street stands and low-end stores, said Ehrlich. The trademark owners aren't aware of any knockoffs that have produced the Tarzan yell - yet. But given the success of the movie and the growing popularity of toys with microchips that make sound, it's a distinct possibility.

Ehrlich said it's impossible to trademark Tarzan's appearance, since he is primarily a literary character. The owners can trademark only individual Tarzan manifestations as they appear in movies and television shows. Thus it was logical to trademark ''The Sound of the Famous Tarzan Yell,'' as first heard in the 1930s Tarzan movies by Metro-Goldwyn-Mayer, he explained. ''It's an important component of the Tarzan character's identity, and one that seems likely would be copied without authorization.''