- Acting for Two: Michael Wisher (Morelli) also provided the voice of Senior Crewman Ranjit, spoken to by Vishinsky over the spaceship's intercom in part four, but was uncredited on-screen. The cod-Indian accent that Wisher used for this scene would most probably be frowned upon today.
- Recycled Set: The Thal spaceship doors from "Planet of the Daleks" are reused for the geology ship exterior, and floor panels and structural components from "The Ark in Space" and "The Mutants" in the ship interior. In particular, a distinctive set of walls with a waffle-like triangular embossing first used in "The Mutants" appear in many serials well into the eighties, and even turned up on Blake's 7 as well.
- What Could Have Been: The original script had Sorenson dying after falling into the pit, but producer Philip Hinchcliffe ordered that this be changed, as he felt it would too grim an ending for "the little ones" and because he saw Sorenson as a victim of the planet's influence rather than an evil man in himself. Instead, a scene was added in which Sorenson is released from the pit and cured of his anti-matter contamination.
- You Look Familiar:
- Michael Wisher makes his final appearance in the series. This was his third appearance in four serials, all broadcast in the same year — he had also played Davros in "Genesis of the Daleks" and Magrik in "Revenge of the Cybermen".
- Prentis Hancock (Salamar) previously played a reporter in "Spearhead from Space" and Vaber in "Planet of the Daleks".
- Frederick Jaeger (Professor Sorenson) and Ewen Solon (Vishinsky) both previously appeared in "The Savages", in which they played Jano and Chal respectively.
Trivia / Doctor Who S13 E2 "Planet of Evil"