The film received mixed reviews at the time of its release. Roger Ebert mocked the "really bad special effects" and the "odd compulsion" of the characters "to jump into the water the very moment they discover it is infested by piranhas." "Variety" wrote that the film was "not without its exciting moments" and noted that the in-jokes for film buffs added "another dimension to a routine potboiler." Gene Siskel of the "Chicago Tribune" gave the film one-half star out of four, particularly objecting to the use of women and children as frequent targets of the attacks. Jill Forbes of "The Monthly Film Bulletin" called it "a quickie from the Corman company that can't decide whether to plump for horror, science fiction or social comedy, and plays safe with something for everyone," and found it "a lot of fun." Marjorie Bilbow of "Screen International" wrote that the film "works as spirited scream and giggle nonsense because there is always something happening and Joe Dante directs with a slyly tongue-in-cheek awareness of the existence of big brother Jaws."