The "flatworm theory" used in the movie (that flatworms can eat the remains of their own kind and absorb their knowledge) is based on actual tests given to flatworms which involved seeing how quickly they would make the "correct" turn at a Y-intersection after eating the previous experimenters. The results were more inconclusive than the film says.
The theatrical poster depicting the film's principal cast is a composite made of various photo shoots for other movies produced by Miramax Films at the time of release: heads belonging to Rose McGowan and Joanna Going have been placed on the bodies of Drew Barrymore and Neve Campbell from a promotional shoot for Pânico (1996), while the image of Liev Schreiber is a flipped version of the same picture of Schreiber on the poster of Pânico 2 (1997). Meanwhile, the image of Ben Affleck is an unused shot from a promotional shoot for Procura-se Amy (1997).
"Affleck, you da bomb in Phantoms, yo!" was a phrase coined by Jason Mewes in O Império (do Besteirol) Contra-Ataca (2001), and has been a popular phrase whenever Ben Affleck is mentioned in media.
Rose McGowan instantly bonded with the elderly Peter O'Toole, whom she later called one of her favorite co-stars. After filming, he invited her to accompany him when he was asked to leave his hand and footprints outside Mann's Chinese Theatre in Hollywood, and she recalls him jokingly whispering to her "darling, let's run away together and start a scandal." Twenty years later, at a convention panel, she referred to him as "one of the great loves of my life."
An adaptation of the novel was originally set to be made in the late 1980s or early 1990s by New World Pictures and Allied Vision Entertainment, but was shelved after New World filed for bankruptcy.