This story includes Louise Fletcher's first guest appearance on Perry Mason. Her second appearance was in The Case of the Larcenous Lady (1960) and her second appearance aired in 1960 - and Louise was the defendant in BOTH, a rarity for this series.
The dramatic incidental music playing during the scene where Perry searches Richard Gilman's car registration to obtain his name is lifted directly out of the soundtrack of "The Day the Earth Stood Still".
The murder victim is never seen alive in this episode.
Per various car registrations in the Series, the same license plate number (NFL 252) has been seen, or will be seen, five times for five different characters. The episodes are The Case of the Bartered Bikini (1959), The Case of the Prudent Prosecutor (1960), this episode (The Case of the Mythical Monkeys (1960)), The Case of the Crying Cherub (1960), and The Case of the Barefaced Witness (1961).
When private investigator Kelton refers to "Lower California", that is Southern California slang for the Mexican province of Baja California.