Director Michael A. Hoey and producer Jack Broder had major disagreements during shooting. Broder changed the title from "Night Crawlers" - the title of the book upon which the script was based - to its current title, which Hoey detested. Broder also had story ideas Hoey disagreed with, and when the "tree stump monsters" arrived, Hoey thought they were ridiculous and refused to shoot them. Broder called in Arthur C. Pierce, who was working on another movie Broder was producing, and had him shoot additional scenes to add the story elements Broder wanted. He also had Jon Hall, who had created the "tree stump monsters", shoot the scenes with them that Hoey refused to do.
Roger Corman was an uncredited producer on the film.
The entire cast and crew were on the point of walking out during production when they learned halfway through filming that the film was going to be released with the title The Navy vs. the Night Monsters (1966).
Shot in ten days.
Mamie Van Doren was cast because she owed uncredited producer Roger Corman another film on her contract. Van Doren was highly dismissive of her work on the film and indeed the film itself.
Producer Jack Broder wanted a 90-minute film so he could sell it to TV networks. The finished product was 78 minutes, so Broder hired Arthur C. Pierce, director of Women of the Prehistoric Planet (1966), to film some additional scenes.
After the movie had completed shooting a producer added the scenes of the island being destroyed by military bombers. There also were close-up shots of the creatures showing off how low budget the effects were.
Michael A. Hoey optioned the original novel and wrote the screenplay in 1959, hoping to emulate the success of The Thing from Another World (1951). He was paid $10,000 for the script.