The original story concept did not depict the aliens with bi-colored skin. One was a devil with a tail and the other was an angel. Episode director Jud Taylor came up with the idea of bi-colored skin shortly before the episode began filming. His original suggestion was that they be half-black/half-white, one color from the waist up and the other from the waist down, but each wearing reversed color schemes. The central idea stuck but the colors were finally separated along the vertical axis rather than along the horizontal.
Bele's totally "invisible" ship perhaps is the most noticeable effect of the biggest budget cut in the original series.
During the filming of Frank Gorshin and Lou Antonio's run sequences, Gorshin and Antonio collided with one another when neither actor knew the other was striding down opposite ends of the corridor. The camera crew hadn't warned them that their scenes were being shot simultaneously.
This was the last episode Robert H. Justman worked on as co-producer. He left the show because of its declining quality and NBC's harsh treatment of it.
This episode represents the last on-screen appearance of the hangar deck in the original series. The shuttlecraft makes one last appearance on the planet set of The Way to Eden (1969).