- Name's the Same: Gene L. Coon got in a bit of trouble due to similarity of the episode to a short story of the same name he had read and forgotten. The research agency spotted it immediately and contacted Fredric Brown's agent, inviting him to "write something for Star Trek". After numerous improvements, Coon's work was sent to Brown, Brown okayed it and was given both money and screen credit.
- Recycled Set: Cestus III was a reused Alamo set from the 1930s; it was later demolished for being unsafe.
- What Could Have Been: In the original script, Kirk and the alien captain's battleground had translucent walls, making it seem as though they were in a giant terrarium.
- You Sound Familiar: Ted Cassidy has his final Star Trek role as the voice of the Gorn. Cassidy had also voiced the antagonist in "The Corbomite Maneuver" and appeared as Ruk in "What Are Little Girls Made Of?".
Trivia / Star Trek S1 E18 "Arena"