Tab Hunter was originally chosen to play Doctor Paul Stewart.
In one draft of the script, the film was to end with the volcano not only destroying Sanda and Gaira, but the lava was to destroy Tokyo and the remaining cells of the two monsters as well.
This film was a sequel to the film Frankenstein Conquers the World. However, all references to the previous film were cut from the American version of this film.
When the film was released in America, Russ Tamblyn had to re-dub his own voice because of a malfunction in the soundtrack.