Gilbert Arthur Grissom (born August 17, 1956), Ph.D. is a fictional character portrayed by William Petersen on the CBS crime drama CSI: Crime Scene Investigation and its sequel, CSI: Vegas. Grissom is a forensic entomologist and for the series' first nine seasons, a CSI Level III Supervisor employed by the Las Vegas Police Department. He appeared in 198 episodes of the original series, where he was succeeded by Raymond Langston (Laurence Fishburne) and later D.B. Russell (Ted Danson). Grissom returned in the sequel series, CSI: Vegas, in 2021. Appearances Series regular Petersen appeared in every episode of the original series' first eight seasons, with the exceptions of: Season 5: "Hollywood Brass" Season 6: "Gum Drops", "The Unusual Suspect" Season 7: "Sweet Jane", "Redrum", "Meet Market" (briefly) Petersen then appeared in the first ten episodes of the ninth season, before departing the main cast. Petersen is part of the main cast for the first season of the sequel, CSI: Vegas. Guest star Following his departure from the main cast, Petersen reprised his role on five occasions. Voice Season 09: "Turn, Turn, Turn" Season 13: "Wild Flower", "Dead Air" On screen Season 11: "The Two Mrs. Grissoms" Petersen also guest starred in "Immortality," CSI's feature-length series finale. Peterson also guest stars as Gil Grissom in the TV series Without a Trace in Season 6, episode 6. Creation Anthony E. Zuiker, who created the show, loosely based Grissom on real life Las Vegas Metropolitan Police Department criminalist Daniel Holstein. Zuiker was "fascinated" by Holstein, who, like Grissom, kept maggots and pig's blood in his refrigerator. Holstein worked as a consultant for the show. Zuiker originally named the character Gil Sheinbaum, but after he became part of the cast, William Petersen requested the name to be changed. Petersen and Zuiker decided to rename the character "Grissom," after astronaut Gus Grissom, of whom Petersen was a great admirer. The name Gil came from one of the actor's hobbies, fishing. In 2000, CBS had bought the pilot script from Zuiker, and Nina Tassler, CBS's head of drama development, passed it on to Petersen, who had a pay or play contract with the network. Petersen has said that many TV shows were offered to him over the years, but he "didn't want to get locked down." However, he was impressed with the complexity of Grissom's character and he decided to audition. It was the second time Petersen