Strother Martin
Having just seen this again for the first time in years, including finally seeing the ending, I have to give Strother Martin a lot of credit. I was very impressed with his performance. He was totally believable, even sympathetic, as the mad scientist, but more than that he apparently put a lot of time and research into his part. He handles the snakes and speaks confidently, exactly as if he had made a career of it. Very professional behaviour by an actor for a part in a movie that was clearly going to be just a drive-in B-flick.
Heather Menzies deserves credit also for her few scenes with snakes - the weakest link was Dirk Benedict, I can't imagine anyone caring about what happened to him.