* Season 1 (1969): David M. Mouser sang the theme song. He was a session vocalist and a mainstay in the Hanna-Barbera sound.
* Season 2 (1970): Neil Diamond sang the theme song. Yes, *that* Neil Diamond! He was at the height of his pop music fame at the time and agreed to sing the theme for a fee of $10,000.
Interestingly, the song remained virtually the same, with only minor changes in the arrangement. It's pretty neat to listen to the two versions and pick out the subtle vocal differences between Mouser and Diamond!