Willis H. O'Brien and Pete Peterson completed a significant amount of the stop motion animation on a table in Peterson's garage.
There are lots of Beatles connections in this movie. Jessie Robbins, who played Aunt Jessie in Magical Mystery Tour, is in a scene drinking tea whilst the radio reports are airing. Norm Rossington, who played Norm in A Hard Day's Night, is one of the men killed in the car when the Beast tosses it into the Thames. Lastly, in the scenes showing a deserted London, in one of the streets is the block of flats the Beatles live in at the start of Help.
US prints do not have Douglas Hickox sharing the directing credit with Eugène Lourié, nor list Leonard Sachs among the cast. The UK version's credits, on the other hand, leave off the entire special effects crew.
The puppet of the behemoth is owned by Dennis Muren.
The helicopters in the movie are a us h-5 dragonfly helicopter and a H-19 with RAF markings.
Many of the sound effects are taken from King Kong (1933), including Fay Wray's scream.
Several clips from the film were used in Knots Landing: Giganticus II: The Revenge (1989), Knots Landing: Dial M for Modem (1989) and Knots Landing: That's What Friends Are For (1989) to represent the fictional Japanese kaiju film "Giganticus".