- Scenes in which the baby Gorgo is driven through London were shot on a Sunday morning, which meant hardly anyone was in the streets. This was then explained away by a "TV announcer" who said that people were told to stay indoors.
- The film was originally set in Japan. Later, the King Brothers and Eugene Lourie thought of setting it in Paris, but that would have meant the monster Gorgo would have had to wade about 100 miles up the Seine to get to the city. Finally, it the location was changed to Ireland and England.
- The original script had no military action indicated. Director Lourie argued that gunfire would kill a beast, no matter how big. The King Brothers wanted guns and they got them.
- Lourie later acquired a 35mm print of the movie for private use and cut out all the stock footage military shots.
- The ending was inspired by Lourie's daughter who cried at the end of the Giant Behemouth when the monster was killed. When this happened, he decided to let both monsters live.
- The concept for the British poster art and lobby cards was that the face of Gorgo was not revealed. The idea is based on the line of dialogue in the film from actor Martin Benson, that Gorgo's name is derived from the Gorgon sisters of Greek mythology. The site of their faces would turn you to stone.
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