A sequel, set in Africa, was planned. However, G-series producer Tomoyuki Tanaka, who had been hospitalized during the film's production, was enraged by the film once he saw it, telling director Yoshimitsu Banno that he had ruined the Godzilla series. Tanaka immediately ordered the filming of a more conventional Godzilla movie (Godzilla vs. Gigan (1972)). Thus, the "Smog" sequel was never made.
This was the first film that featured Kenpachirô Satsuma to wear the Smog Monster suit. Though small in stature, Satsuma was quite strong for his size, and was the only one capable of supporting the 300 pound suit for long periods of time. (Though there were some wire works to help support.) Satsuma then went on to wear the Gigan costume for the next two films. After a break of over 10 years, he would be asked to wear the Godzilla costume for Godzilla 1985, and would continue to wear it through the Heisei series, and retired after Godzilla vs. Destroyah in 1995.
Over the decades, fans have kept wondering what the strange orbs that Godzilla rips out of Hedorah's dried-out body actually are. Most think that they are either eggs or his eyes, and there's a common joke that they are his testicles. According to the director, those are meant to be Hedorah's eyes, which he considered the most important part of a person's body. The movie also has a running theme of eyes being injured.
When Godzilla chases down Hedorah near the end of the film, the director originally shot two different scenes. One had Godzilla chasing Hedorah on foot, the other had Godzilla flying after him. The flying scene was the one used in the final cut of the film, because the director thought a comical scene was needed to lighten up an otherwise dark film.
This was the last Godzilla film to be released by American International Pictures (AIP) and dubbed by Titra Productions. The remaining Godzilla films from this decade were released by Downtown Distribution and/or Cinema Shares, and simply used edited versions of Toho's international English prints.
The original Japanese trailer promoted this as "Promising young director Yoshimitsu Banno debut title." In reality, producer Tomoyuki Tanaka and the studio were so disappointed in the finished film that Tanaka banned Banno from ever working on another Godzilla film for as long as Tanaka lived. Ironically, in the years following Tanaka's death, Banno acquired Godzilla's film rights and planned to produce an IMAX short film entitled Godzilla 3-D to the Max. When Banno could not acquire funding for the film, he approached Legendary Pictures on behalf of Toho and discussed production of a new feature film. Ultimately, Godzilla (2014) was produced from this deal, with Banno acting as executive producer.
There are differing accounts on who came up with the movie's anti-pollution message and dark imagery. Director Yoshimitsu Banno says it was him, as he wanted to make the Godzilla series relevant again by inserting social commentary into the story. Special-effects director Teruyoshi Nakano on the other hand claimed that Banno was trying to make the movie appeal to kids, and that the anti-pollution message and the darker scenes were his ideas.
For the first time, the American International Pictures (AIP) dub of the movie translated its theme song, "Bring Back Nature". The English song, re-titled "Save the Earth", was performed by then-upcoming singer Adryan Russ. Russ claims that for a long time she had considered her involvement with a Godzilla movie an embarrassments, but upon learning of the movie's cult status, she completely changed her mind. The other official English dubbing track for the film, produced by Toho, keeps the song in Japanese.
Though it's commonly said that producer Tomoyuki Tanaka was furious at director Yoshimitsu Banno after viewing the finished film, Banno claims that people over-exaggerate the story and that Tanaka's reaction to the movie, while negative, wasn't nearly as heated.