The olive fruit fly was detected in Southern California in October 1998. This nonnative pest, which can render fruit unmarketable, has since moved throughout California and is now believed to be present wherever olives are grown in the state. Seasonlong trapping of adult flies in the San Joaquin Valley, and Ventura and Santa Barbara counties in 2001 and 2002 showed similarities in seasonal phenology, but also differences primarily due to varying temperatures and fruit availability. In the San Joaquin Valley, fly activity declined as maximum daily temperatures rose above 90?F, but increased when temperatures were between 70?F and 85?F. On the Southern California coast, the combination of available, susceptible fruit and moderate climate throughout the year may allow continuous reproduction of OLF with six or perhaps even seven generations per year.