During 2003, West Nile virus [WNV] invaded rural Imperial County, California, where it was detected by virus isolation from Culex tarsalis mosquitoes and seroconversion of sentinel chickens. WNV then dispersed to the north shore of the Salton Sea and then jumped to suburban/urban habitats in Riverside, San Bernardino and eastern Los Angeles where it was associated with large communal crow roosts. In urban Los Angeles WNV was tracked by virus isolation from pools of Cx. p. quinquefasciatus and testing dead birds reported by the public. Virus activity subsided during winter, even though temperatures in southern California remain mild. During 20004, WNV was isolated from dead crows throughout winter [Feb - Mar] and then amplified markedly in the Whittier Dam area of eastern Los Angeles. WNV also over wintered in SE Coachella Valley, but not in Imperial County. The pattern of virus amplification and dispersal during the summer of 2004 will be discussed.