Filming the movie's monster was a bit of a tongue-in-cheek affair. For shots that wouldn't show the monsters feet, Wade Popwell, the actor in the monster suit, would usually be wearing shoes. So unless we're seeing the monsters feet in a shot - it would typically be wearing tennis shoes on location.
In an interview, writer Ron Kivett stated that the inspiration for the movie came from an article he read about a species of 'walking' catfish that could live both in water and on land. The article theorized that the fish were possibly a mutation, and Kivett thought it was an intriguing idea for a story. Doctor Leopold refers to the same species of fish in his endless voice-over narrative in the movie.
Originally the film was suppose to have scenes of gigantic catfish destroying the countryside but footage of the walking catfish on miniature landscapes proved to be too hokey. However one shot still made it into the film as one catfish can be seen squirming next to a miniature fence during one sequence.
Director Don Barton ran an ad in a local paper to find someone for the role of the monster. The ad read: "Wanted: 6'5'' or taller male to play the role of monster in horror movie. Must be experienced swimmer, scuba diver. Acting Ability not required!" Barton said a total of ten people responded to it.