PRIMUS is the band 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?") He also appears on the intro when the show airs in syndication.
Les Claypool
They made an appearance in the Season 2 episode "Chef Aid" playing at the concert to support Chef.
PRIMUS performed "South Park Theme" and "Mephisto and Kevin" on Chef Aid: The South Park Album.
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 "Tooth Fairy Tats 2000".
Les Claypool appeared in Chef Aid: Behind the Menu, in which he talked about Chef's influence on his career.