As the film was being finished up, producer Roger Corman felt that the picture needed more sex, and ordered scenes shot that showed the "humanoids" attacking - and, of course, ripping the clothes off of - other nubile young women. Director Barbara Peeters refused to shoot the scenes, protesting that they were inserted purely to show gratuitous nudity. Corman fired her and hired another director to shoot the additional footage. However, most of the re-shot Humanoid attack scenes were deleted from the final print of the movie.

There are only three Humanoid creatures seen together on-screen in the same frame throughout the movie. The production really only had one fully functioning Humanoid costume and two others that could only be shot from certain angles because they weren't as convincing. Use of such angles, as well as the editing, would help to create the illusion that there were much more Humanoids than just three for the climactic carnival massacre.

Director Barbara Peeters once said that the "humanoids" were originally suppose to be played by the film's stunt men. Unfortunately, the stunt men found the monster suits to be too "goofy-looking" and refused to wear them. Additional actors had to be hired to perform as the monsters.

Actress Ann Turkel once said why she chose to do this film: "It was an intelligent suspenseful science-fiction story with a basis in fact and no sex". However, with the filming of additional footage, the sex content changed.

One scene of the Humanoids attacking a victim was shot when the temperature was cold enough to turn the actress's lips blue.

The green monster footage and fairground destruction were re-shoot material filmed during several very rainy nights in Malibu. Gore video guru Larry Wessel has a bit part murdering a Humanoid in the crowd scenes.

According to Movie News (Australia) magazine, after her discovery of the explicit additional footage, "Ann Turkel was so outraged that she asked Hollywood's Screen Actors' Guild to try and stop 'Monster' [Humanoids from the Deep] from being released".

A sequel was planned and set to be released in 1991 by 21st Century Film Corporation and was to be produced by Menahem Golan but was never materialized.

According to the Film School Rejects website, this picture was "one of the earliest films to deal with genetic modification".

The picture was filmed during October, November and December 1979.

Much of the salmon festival carnival footage was reused in the movie's 1996 TV remake Humanoids from the Deep (1996).

James Horner composed the score in only fourteen days.

The film was made and released about sixteen years before it was re-made for television with Humanoids from the Deep (1996) but the second time the levels of sex and violence for the remake were greatly toned down.

One of a cycle of 1980s and mid-late 1970s movies that got made after the box-office success of Jaws (1975). The films include that movie's three sequels, Jaws 2 (1978), Jaws 3-D (1983), and Jaws: The Revenge (1987), as well as Orca (1977), Piranha (1978), Tentacles (1977), Killer Fish (1979), Barracuda (1978), Tintorera: Killer Shark (1977), Blood Beach (1980), Piranha II: The Spawning (1981), The Last Shark (1981), Up from the Depths (1979), Screamers (1979), Devil Fish (1984) and Mako: The Jaws of Death (1976).

The name of the small Californian fishing village was Noyo. The locale is a real life place it being an unincorporated community in Mendocino County, California.

The production was originally filmed under the title "Beneath the Darkness" in the hopes that it would infer a classier type of story and therefore attract the kind of big name cast the filmmakers wanted.

Joe Dante, who had at the time recently directed Piranha (1978), turned down directing the movie.

Showbusiness trade-paper Variety said of this movie in 1980 that "with Humanoids from the Deep (1980), Roger Corman comes full circle back to his very first film as a producer, Monster from the Ocean Floor (1954) [1954]. Despite costing 100 times as much, new pic has similar premise and same raison d'etre, that of pocketing a profit from drive-in dates" with the picture having "more nudity and gore than carried by any exploitationer in recent memory".

The height of the monster sea creature humanoids from the deep, as mentioned in the film's storyline, was said to be around six and a half to seven feet tall or two meters high.

Final theatrical feature film [to date, 2019] directed by Barbara Peeters.

All of Linda Shayne's scenes as Miss Salmon were shot by the second unit.

"Humanoids from the Deep" was released within a year of the similarly titled picture "The Humanoid" [The Humanoid (1979)].

One of two Creature from the Black Lagoon (1954) type films featuring amphibious creatures released within a year of each other around 1979/80. The other movie was Screamers (1979) [The Island of the Fishmen].

This movie and Portrait of a Hitman (1979) were the first films that actress Ann Turkel made without her husband Richard Harris. They had previously made three films together.

Star Billing: Doug McClure (1st), Ann Turkel (2nd) and Vic Morrow (3rd).

Debut theatrical feature film of actors David Strassman, Breck Costin and Cindy Weintraub.

The movie's ending was a direct rip-off of Ridley Scott's Alien (1979). Allmovie states "the film features an Alien (1979)-inspired shock ending".