High Quality Content by WIKIPEDIA articles! Dr. Gilbert "Gil" Grissom, (Ph.D.) is a fictional character on the CBS crime drama CSI: Crime Scene Investigation, 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 1 to 9, when Petersen left the show as a regular, and was replaced by Laurence Fishburne as Dr. Raymond Langston (M.D.).