She of course, was replaced by (https://forums.stevehoffman.tv/threads/tv-stars-who-were-fired.589666/page-9#post-16166113) Eartha Kitt in the third and ultimately final season. The official reasoning was that Julie Newmar was making a film called MacKenna's Gold (https://en.wikipedia.org/wiki/Mackenna%27s_Gold). But the entire notion that her filming schedule for MacKenna's Gold would've conflicted with being able to appear on Batman has since come into question (https://www.66batman.com/forums/viewtopic.php?t=860). One theory that's out there (https://forums.stevehoffman.tv/threads/tv-stars-who-were-fired.589666/page-9#post-16164618) states that Julie wasn't invited back out of concern that she would've upstaged Yvonne Craig (https://www.66batman.com/forums/viewtopic.php?t=7252), since Julie was significantly taller than her.