In an interview, star Marshall Thompson recalled that director Arthur Crabtree didn't really want to direct the film--he thought sci-fi was "beneath" him--and often didn't show up for work. Eventually, according to Thompson, Crabtree walked off the picture, and Thompson himself finished directing it.
The Criterion Collection DVD has a hidden gag on it added by the authors. If you check the disc name/ID on a PC through various means, the ID reads AFIENDINNEEDISAFIENDINDEED, i.e. "A fiend in need is a fiend, indeed".
A publicity stunt went somewhat wrong in New York City. The Rialto Theater in Times Square featured a sidewalk promotion for the film--one of the prop "brain creatures" was displayed in a cage on the sidewalk outside the theater, wired for sound and motion. However, the crowd it attracted grew so large that they were snarling pedestrian and vehicle traffic, and the police demanded that it be removed.
The film created a public uproar after its premiere at the Ritz Theater in Leicester Square. The British Board of Film Censors had already demanded a number of cuts before granting it the "X" certificate.
This 1958 British production was based on "The Thought-Monster" by Amelia Reynolds Long, which was published in the classic American pulp magazine "Weird Tales" in 1930. The story was originally submitted by Forrest J. Ackerman in the late 1950s to American-International Pictures, which turned it down. AIP producer Alex Gordon thought that his brother, New York-based producer Richard Gordon, might be interested in it, so submitted it to him. Richard Gordon liked it, and his company Amalgamated Productions eventually produced it in England.