Katahdin, the mutant bear-monster, is portrayed by 7"2' actor/stuntman Kevin Peter Hall who went on to play other memorable man-in-a-suit monsters of the 80s including the giant egg-headed alien in Without Warning (1980), the titular creature in Predator (1987) and Predator 2 (1990), and Bigfoot in Harry and the Hendersons (1989).
According to director John Frankenheimer during post-production the studio demanded that the film be cut from an R rating down to a PG. Frankenheimer felt this damaged the film, destroying the scariness it had created.
According to the Science Fiction, Horror and Fantasy Review, "David Seltzer has taken the basics of Prophecy (1979) from a real-life apocalypse - the environmental disaster in the Japanese city of Minimata, which came to light in 1958 where it was discovered that mercury waste being dumped into a nearby river from a chemical plant had caused severe mutations and neurological degenerations among the locals. The effects of this consisted of loss of muscular control, vision and hearing, followed eventually by insanity and paralysis."
Filmed in British Columbia in 1978, this movie marked the beginning of the "Hollywood North", the major start to the development of a massive film production business in Vancouver and other parts of the province of British Columbia, Canada. Since then hundreds of movies and TV series which suppose to take place somewhere in the US have been filmed in the Canadian province.
Talia Shire's involvement in this film meant that Sylvester Stallone had to rewrite the final scenes in Rocky II (1979).