Producer Ed Nelson, who also stars, created the parasites himself using little wind-up toys covered with fur from an old coat and pipe cleaners for antennae.
The producers of this movie were sued by Robert A. Heinlein, who claimed the plot stole several elements of his novel, "The Puppet Masters". The suit was settled out of court.
The film music used throughout this movie is credited to "Tom Jonson." In fact, virtually the entire film score is made up of uncredited recordings of Dmitri Shostakovich's Fifth Symphony. Neither Shostakovich, nor his publishers ever received royalties for this music piracy.
Some sources have claimed that future exploitation director Jack Hill acted in this film. "Jack Hill" was actually the name being used by veteran actor Cornelius Keefe (who plays Senator Walter K. Powers) for his film credits during the 1950s.
His role as Senator Walter K. Powers became the last role for Cornelius Keefe, ending a career of almost 35 years starting in silent films. He passed away in 1972.