The noises made by the dolphins - eventually becoming recognisable speech - were provided on the soundtrack by the film's scriptwriter, Buck Henry (who was uncredited in this role).
Lamar Boren was responsible for filming all underwater scenes.
The dolphins who appeared in this film were trained by Peter Moss, Kathy Trout and Erica Abt.
Dr. Jake Terrell: [on returning to the island]: I go away for one lousy afternoon and the whole place falls apart!
Dr. Jake Terrell: [discussing dolphins]: Why, as mammals, were they compelled - or did they decide - to leave the land and return to the sea?
Harold de Milo: Are you a blackmailer, Mr. Mahoney?
Curtis Mahoney: Goodness, no! I'm just an average guy with an above-average curiosity.
Robert Merle's novel was originally called Un Animal Doue De Raison (An Animal Endowed With Reason) and was originally published in French in 1967. It was an international best-seller, appearing in English translation in 1969, but making a film of it proved very difficult. Roman Polanski was working on it with the writer James Salter at the time of his wife's murder; he then dropped the project.
This film, like Robert Merle's original novel, was inspired by the researches of Dr. John Lilley. He threatened the makers of the film with a lawsuit, although it came to nothing (perhaps because the film was a box-office flop).