The original Danish-language version was directed by Danish director Poul Bang and released in Denmark on February 25, 1961.
The American version, which was in English with a nearly identical cast, was directed by the film's American producer-director Sidney W. Pink; this version was initially deemed virtually unreleasable by American International Pictures and had to be extensively reworked by the film's Danish-American screenwriter, Ib Melchior, before being finally released in America in 1962. Pink was angry at the changes and wound up in a legal dispute with AIP. After Pink and others viewed the English-language version, the lawsuit was dropped.
Danish miners Svend Viltorft dig up a section of a giant reptile's tail from the frozen grounds in Lapland, where they are drilling. The section is flown to the Danish Aquarium in Copenhagen, where it is preserved in a cold room for scientific study. But due to careless mishandling, the room is left open and the section begins to thaw, only for scientists to find that it is starting to regenerate.
Professor Otto Martens, who is in charge of the Aquarium, dubs the reptilian species "Reptilicus" (upon a reporter's suggestion) and compares its regeneration abilities to that of other animals like earthworms and starfish.
Once fully regenerated from the tail section, Reptilicus goes on an unstoppable rampage from the Danish countryside to the panic-stricken streets of Copenhagen (including one of its famous landmarks, Langebro Bridge), before finally being killed with poison by ingenious scientists and military officers.
However, the monster's foot is not destroyed and sinks to the bottom of the sea. The movie is left open-ended, with the possibility that the foot could regenerate.
- Carl Ottosen as General Mark Grayson
- Ann Smyrner as Lise Martens
- Mimi Heinrich as Karen Martens
- Asbjørn Andersen as Professor Otto Martens
- Bodil Miller as Connie Miller (Danish version)
- Marla Behrens as Connie Miller (American version)
- Bent Mejding as Svend Viltorft
- Povl Wøldike as Dr. Peter Dalby
- Dirch Passer as Petersen
- Ole Wisborg as Captain Einer Brandt
- Denmark's first and (so far) only "Monster" movie.
- Filming took place in several locations in Denmark, including Copenhagen, Sjælland, and Jylland.
- Several versions were filmed, the original film was filmed using the native Danish language and the second was filmed using the English language. Each version of the film featured the same actors speaking in English with the exception of Bodil Miller who was replaced by actress Marlies Behrens since the Danish actress could not speak English. However the English version of the film was heavily edited and the actors voices dubbed over by American International Pictures for its release in the United States.
- As Denmark's first and only giant monster film, this film has a cult following in its home country. Sidney Pink attempted to produce a remake of the film in 2001, due to the box office hit of Godzilla in 1998, before his death in 2002.
- The film received mostly negative reviews from American critics, it currently has a 29% "Rotten" rating on review aggregator website Rotten Tomatoes, based on 7 critical reviews and a score of 37% from users.
- Though like Gorgo, and not officially part of the culture, the film is considered to be an completionist addition to fans of Kaiju, a Japanese film genre that features monsters, usually attacking major cities and engaging the military and other monsters in battle.
- A novelization of the film was released in paperback at the time of its original release by Dean Owen. It was peppered with gratuitous sexual content entirely irrelevant to the film's story. Director/screenwriter Sid Pink was so enraged by this soft-porn elucidation that he sued the publisher, Monarch Books.
- In 1961, Charlton Comics produced a comic book based on the film. Reptilicus lasted two issues. After the rights had lapsed, Charlton modified the creatures look and renamed it (and the comic) Reptisaurus.
- Sidney Pink attempted to produce a remake of the film in 2001, due to the box office hit of ''Godzilla'' in 1998, before his death in 2002.
- Although it was shot in Denmark, producer Sidney W. Pink had the dialog spoken in English. When the finished film was turned over to American International, Pink was informed that the dialog would be re-looped. AIP head Samuel Z. Arkoff felt that what he called the "sing-song Scandinavian accents" would have U.S. audiences laughing. Pink objected and threatened to file suit. After Pink had other industry professionals view his version of the film, the lawsuit was dropped and AIP released the film in an edited and re-dubbed version.
- Because Dirch Passer was hugely popular as a comedic actor at the time this was filmed, the producers at one point were going to call the film "Dirch and the Dragon".
- Many of the elements in the script for this film were originally intended for the aborted "Volcano Monsters" project, also written by Ib Melchior.
- In the scene where Reptilicus eats a farmer, one of the farmer's sons is played by Dirk Melchior, the son of writer Ib Melchior (who was 12 years old at the time).
- Nora Hayden originally was slated to play Lise. However, she quit the project when she discovered that she would only receive second billing.
- Writer Ib Melchior dubbed several voices in the English-language version, including a reporter, a police officer and a mayor.
- Argentinian music group Los Twist composed a song about this movie for their album "La máquina del tiempo''.
- A clip of the movie was featured in the South Park episode Cancelled.
- There is an inside joke among toy collectors about the LJN Toys' Dungeons and Dragons "goofy-looking" Bronze Dragon figure from the 1980s and its resemblance to Reptilicus.