In a move apparently aimed at protecting CBS's most lucrative franchise, William Petersen, who's leaving Csi Crime Scene Investigation this season is being replaced by film star Laurence Fishburne, the network announced Monday. It said that Fishburne will be introduced on a forthcoming show as a college professor/criminologist who studies the reasons people turn to crime and who is asked to consult on a case with the CSI team. He is subsequently asked to join up with them.