The surprise box office success of this film would inspire a huge amount of animal-themed horror films in the early 1970s.
According to Ernest Borgnine's autobiography, the studio offered him a choice of either a higher salary or a percentage of the box office. Borgnine chose the higher salary.
According to an issue of "TV Guide" released in 1971, the year of the film's release, Ernest Borgnine had to overcome a fear of rats prior to filming.
Bruce Davison had a rat placed on his shoulder as part of his audition for the part of Willard Stiles.
Bruce Davison and Sondra Locke were dating in real life at the time the film was being made in 1970. They never appeared as a couple in public because Locke was still legally married.