Second-unit director David Sharpe recalled an incident that happened on the nightclub set: "We were shooting some action stuff . . . which featured lions, tigers, elephants, and apes in cages around the table area. We had trained lions to run from one side of the set to the other. One of them broke loose, jumped onto the top of his cage and grabbed the trainer by the throat. I was in the camera cage. I pushed the iron door open, raced across the set and punched the lion in the face. I guess I shocked him so badly he let go, turned tail and ran into his cage."
Though Willis H. O'Brien gets top special-effects billing, Ray Harryhausen actually did 85%-90% of the stop-motion animation for this film, although the animation is based on O'Brien's designs and storyboards.
One of the few films to ever feature a real gorilla. An infant gorilla was filmed in Africa to be used as Joe when he was a baby in the beginning of the film.
Look for these unbilled stars of the future (and past): Ellen Corby (The Waltons (1972)) in the orphanage scene; William Schallert (The Patty Duke Show (1963)) as the gas station attendant; Kermit Maynard, singing cowboy, as Red in the roping scene; Jack Pennick, perennial John Ford extra, as the truck driver whose truck the heroes steal; and Irene Ryan (Granny in The Beverly Hillbillies (1962)) in the nightclub scene.