Gilbert Arthur "Gil" Grissom, Ph.D. is a fictional character on the CBS crime drama , portrayed by William Petersen. Grissom was a forensic entomologist and the night shift supervisor of the Clark County, Nevada CSI (forensics) team, investigating crimes in and around the city of Las Vegas. He was the show's protagonist from seasons one to nine, when Petersen left the show as a regular, and was replaced by Laurence Fishburne as Raymond Langston, M.D.. He appeared in 198 episodes.