- Acting for Two: William Shatner does this a lot, but his brief appearance as the corpse of Sam Kirk marks the only time that he plays a character who is not either James T. Kirk or someone (or something) mimicking or inhabiting the body of James T. Kirk.
- Deleted Scene: Viewers of the episode as aired are often left with the question of what happens to Kirk's nephew. A scene was shot, but cut, explaining this: Sam Kirk's business partner has offered to adopt Peter, who chooses to live with him and stay on Deneva rather than move to Earth to live with his grandmother (whether this is also Kirk's mother is not stated).
- Promoted Fanboy: Kirk's yeoman in the episode is portrayed by Maurishka Taliaferro, who was a popular and sought-after model in the 1960s. As she was also a fan of the show, she was able to talk the producers into giving her a part.
- What Could Have Been: The first draft script did not feature Kirk's brother and his family. In that version, a Denevan woman named Aurelan was in love with a young man named Kartan, who was brought aboard by Spock so they could study the organisms further. Aurelan's brother Noban was the one who flew his ship into the Sun to destroy the creatures. Aurelan and her father were not infected, and helped the Enterprise crew in their research.
- In the novelization, they find the parasite's home planet and destroy it rather than using ultraviolet light. This was based on an earlier script draft that had The Enterprise destroy Deneva and all of its inhabitants.
- Working Title: Operation: Destroy.
Trivia / Star Trek S1 E29 "Operation: Annihilate!"