 After two months of pre-production, filming began on 2 January 1967 with Desmond Saunders as director.Although "The Mysterons" was the only episode of "Captain Scarlet" that Saunders directed, he served as "supervising series director" for the rest of the production.The incidental music was recorded by series composer Barry Gray in a four-hour studio session held on 16 March 1967, where it was performed by an ensemble of 16 instrumentalists.