Summary: The California Mosquito-borne Virus Surveillance and Response Plan was developed to meet several objectives. Specifically, the Plan: -Provides guidelines and information on the surveillance and control of mosquito-borne viruses in California, including West Nile, St. Louis encephalitis, and western equine encephalomyelitis viruses; -Incorporates surveillance data into risk assessment models; -Prompts surveillance and control activities associated with virus transmission risk level; -Provides local and state agencies with a decision support system; and -Outlines the roles and responsibilities of local and state agencies involved with mosquito-borne virus surveillance and response.This document provides statewide guidelines, but can be modified to meet local or regional conditions.