PRIMUS is an American alternative metal band from San Francisco that composed and performed the South Park theme. The frontman of the group, Les Claypool, sings the lyrics. Les Claypool is somewhat a South Park character as well, with a few appearances on the show. He appears in the opening, singing the theme song in most of the episodes of the show from seasons one through four ("Cartman Gets an Anal Probe" to "Do the Handicapped Go to Hell?")
PRIMUS' song "John the Fisherman" was played in the episode, "Guitar Queer-O". While studying the nature of existence, Kyle has a psychedelic experience while the song "Welcome to This World" is played in "The Tooth Fairy Tats 2000".
Les Claypool can be seen in the South Park intro wearing a red and white striped shirt, a white hat with red trimming, blue jeans, and blue shoes.
As the band appears in "Chef Aid", PRIMUS can be seen wearing yellow and orange duck costumes with blue and green round graduation caps. Les Claypool's bass is pink with green and yellow stripes, hence "Rainbow Bass". The guitarist's (Larry "Ler" LaLonde) guitar is black with green trimming and the drummer's (Bryan "Brain" Mantia) drums are white and blue with black trimming.
- "Chef Aid" - Appearance as one of the Chef Aid bands.
- "The Tooth Fairy Tats 2000" - Two of their songs played during Kyle's trance.
- "Guitar Queer-O" - Performed one of their songs on Guitar Hero.