Godzilla's famous/infamous "victory dance" after defeating King Ghidorah on Planet X was based upon the "shê" gag pose, a trademark of the mischievous character Iyami from the manga "Osomatsu-kun," created by famous Japanese cartoonist Fujio Akatsuka. Iyami would go into this pose every time he is surprised, and scream "shê!" This has become a famous pose in Japanese popular culture. Yoshio Tsuchiya (the actor playing the Controller of Planet X) was a big fan of the "Osomatsu-kun" comic-strip, and asked special effects director Eiji Tsuburaya if Godzilla could do a "shê" pose. Having already given Godzilla a more humorous and playful side, Eiji agreed, cleverly incorporating the pose into a "victory dance" for the monster. However, director Ishirô Honda was not impressed. After some debating, it was ultimately left in the final print.

Nick Adams is speaking English throughout the Film, despite the fact he seems to be conversing with the other actors, who are clearly speaking Japanese. Adams was simply dubbed over into Japanese for the Japanese release. Conversely, when the film was finally released in the US, Nick Adams voice is the only voice not dubbed over. The Japanese speaking actors are dubbed over into English.

Mothra was slated to appear in this film, but for budgetary reasons, she was dropped from the film, as were all references to her in the original script.

The effect of the water seemingly exploding when the flying saucers emerge from the lake was achieved by filling a thermos full of carbon dioxide and exploding it underwater.

Godzilla's design was tweaked again after Ghidorah, the Three-Headed Monster (1964). This design made Godzilla look less menacing and more friendly. The head was shorter and more round, the eyes were larger and more movable, the dorsal fins were a little smaller, and the tongue was much larger. In addition, the suit was made wider under the arms to allow more movement for the actor. However, the effect made the costume appear too "baggy."

The design of the P-1, especially the nose cone, was based on the rockets used for the Gemini space program, which had just started as filming for this picture commenced.

The new Godzilla suit made for this film, called the "Daisenso-Goji," possessed a somewhat friendlier look than the more menacing previous suit, the "Mosu-Goji" (first used in Mothra vs. Godzilla (1964)), considering that the previous film, Ghidorah, the Three-Headed Monster (1964), was the beginning of Godzilla's "hero" status (albeit still a reluctant antihero). This suit would "star" in only one more Godzilla film, 1966's Ebirah, Horror of the Deep (1966), during filming of which Tsuburaya Productions took the head of the suit and fastened it to the body of the Mosu-Goji suit for use as the monster Jirass in the TV show Ultraman: A Special Effects Fantasy Series (1966). In that event, a new head had to be made for the headless Daisenso-Goji suit in short order, creating a strangely bug-eyed stare with an elevated eyebrow line (thus becoming the "Nankai-Goji"). Since then, the Nankai-Goji suit would be a stunt/water suit for all subsequent Godzilla films up to its final appearance in Godzilla vs. Hedorah (1971) in 1971. (In that film, apart from the water scenes, this suit was used for the scene where Godzilla was being covered in sludge by the monster Hedorah.)

When this film was released in the US in 1970, it was released on a double bill with The War of the Gargantuas (1966). That film was a sequel to Frankenstein Conquers the World (1965) which stared Adams. Originally, Adams was signed to that film too, however, Russ Tamblyn was eventually cast in his place after his death.

After the enormous success of the U.S. release of Ghidorah, the Three-Headed Monster (1964), some publications ran articles and photographs of this sequel as an upcoming release under the title "Invasion of Astro-Monster". However, this film was not released in the U.S. until the summer of 1970 under the title "Monster Zero".

This film wasn't released in the US for five years, perhaps because of the death of star Nick Adams. Another possibility is that producer Henry G. Saperstein, who picked up the US distribution rights, had had a falling out with his previous business partners, Samuel Z. Arkoff and James H. Nicholson of American-International Pictures. Eventually, Maron Films distributed the film in the US.

In the scene where Glenn and Fuji meet with the Planet X Controller, Glenn refers to Rodan and King Ghidorah by their Japanese names, Radon and Kingughidorah.

For some close-ups of Godzilla's foot stomping on houses, a 1/10 scale model on a metal rod was used. Although there are several shots of this in the final cut, the prop was a bit of a head-ache for the effects crew. On the Japanese DVD of this film, there is blooper footage of the foot coming loose and bouncing away several times.

In Germany, this movie was released under the title "Befehl aus dem Dunkel" ("Order out of the Dark"), originally the title of a Sci-fi novel by German author Hans Dominik, published in 1933. Although the movie's plot has nothing in common with this book (which prefigures the political effects of a mind reading device), it was even billed as "based on Hans Dominik" and the author's (who had died in 1945) name was widely used in promotion.

After its initial U.S. showings, the film was advertised in some venues under the title "Invasion of the Astros," although the actual prints used still carried the "Monster Zero" title.

This film is part of the Criterion Collection, spine #1,000.

While this was still the era in which Godzilla films had large budgets and crew, it does mark was the first time a Godzilla film used a noticeable amount of stock footage from previous movies. In this case almost 2 minutes culled from MOTHRA and RODAN.

Japan Import DVD has Super-8mm version (Japanese Audio), as a special feature.

Unlike the other American actors who acted in Toho's science fiction films, Nick Adams had no contempt for his work and expressed genuine interest in the hard work of the special effects staff and befriended much of the cast.

This marked the first of three series appearances for Akira Kubo.

During the Godzilla Ghidorah battle towards the end of the movie you can see the New Kirk Motors building from Mothra.

Criterion had intended to release this film on LaserDisc back in the 90s.