Director Saul Bass has said in interviews that the studio tampered with the film during post production against his wishes.

According to actor Michael Murphy, director Saul Bass was very worried about British actress Lynne Frederick being able to sound suitably American for the film, since her character was supposed to be from Arizona. Bass made her run her lines over and over while he listened for any hint of a British accent.

There is footage from the original discarded ending that is used in the promotional trailer for the film.

According to actor Michael Murphy, actress Lynne Frederick was required to wear a sort of corset during filming so that she would appear more flat-chested. Kendra, the character she played, was supposed to be about 16 years old (four years younger than she really was at the time), and the director wanted to minimize her voluptuous figure.

Science Fiction writer Barry N. Malzberg wrote a novel based on Mayo Simon's original script. As a result, the ending of the novel differs from the film.

After working with actress Lynne Frederick in No Blade of Grass (1970) four years prior, Nigel Davenport personally recommended Frederick for the role of Kendra.

The computer console shown at 39:43 into the movie is the MCB44 Display Unit of an Elliott 903 (1963). Another computer visible is a GEC 4080 (1973).

Linda Blair was considered for the role of Kendra. When she auditioned for the film, she impressed director Saul Bass and was asked back to several callbacks. She even went as far as doing a screen test for the part. The producers and director ultimately decided that she was too young, plus the film was also on a limited budget and could not afford the big star's salary at the time.

The original ending of Phase IV was a montage of surreal, haunting images that imply the possibility of the ants conquering mankind and ruling the earth. It hints that "Phase IV" may be a kind of evolution wherein mankind is enslaved and the ants become the supreme rulers. Several of the eerie images allude to the art of Salvador Dalí and René Magritte. The studio cut much of this sequence in favor of shortening the length of the film. This footage still exists and has been shown during special screenings of 'Phase IV', however it has yet to be commercially released.

Body count: 4.