The rape scene had nearly a minute and a half deleted from the final cut of the film before release and, to this day, remains lost or undiscovered. The reasoning for the heavy editing was because Lynne Frederick (who was one of the rape victims in the scene) was only 15 at the time and questions of the usage of a body double for Frederick remains a controversial question to this day.

Lynne Frederick and Nigel Davenport reunited four years later in another science fiction film Phase IV (1974). Because Davenport had gotten along so well with Frederick in their first film together, he personally recommended Frederick for a role in Phase VI when a suitable female actress couldn't be found for the desired role.

First film of Christopher Neame.

Debut film of Lynne Frederick. She beat out over 200 other girls despite having no previous experience in theater, commercials, or film.

According to actress Wendy Richard, who played a supporting role: "It came from a brilliant book, but Cornel Wilde, God rest his soul, I don't think he did it justice when it came to the screenplay. He seemed to go over the top and get some bits of egg on his face."

By all accounts, Nigel Davenport and Lynne Frederick got along beautifully. Frederick later recalled that Davenport would often come around her quoting Shakespeare.

Being Lynne Frederick's first film, it was also her first time away from home. Frederick's mother, Iris, personally asked her daughters older co-star, Wendy Richard, to look over fifteen year old Lynne. In her autobiography, Richard recalled that watching over Lynne was an arduous experience due to Lynne's "headstrong personality".

The last film shot at MGM's Borehamwood Studios. Actress Wendy Richard joked, "I successfully closed down Borehamwood Studios.

According to Cornel Wilde, the childbirth scene was real. It was done through the arrangements of a pregnant mother and her husband. Filming the moment took place in a nearby hospital and the props and wardrobe were properly sterilized. All expenses for the production of the scene went to the mother and Cornel Wilde delighted the couple by giving them a 16mm copy of the sequence.

According to Cornel Wilde in an interview on November 24, 1970, the reason why he did not cast any well-known actors in the film was because he wanted the feeling to be more of a "happening" rather than a movie.

According to the book Omni's Screen Flights/Screen Fantasies, Cornel Wilde wrote an alternate ending taking place one year later in which a little girl finds a single blade of grass growing in a barren field, signifying hope. But because he didn't have the budget for it, the ending was scrapped.