Olive fruit fly, Bactrocera oleae (Gmelin), was monitored with adult captures by season and trap type, and was related to fruit volume and nonharvested fruit to elucidate the occurrence of the newly introduced pest in California. The highest numbers of adults captured in ChamP traps in olive trees, Olea europaea, were in October in an inland valley location, and in September in a coastal location. Comparisons of trap types showed that the number of olive fruit fly adults captured in Pherocon AM traps in a commercial orchard was significantly greater than in ChamP traps. A significantly greater number of females were captured in Pherocon AM traps with bait packets and pheromone lures than traps with pheromone lures alone, while a significantly greater number of adults and males were captured in traps with pheromone lures alone. Significantly more adults were captured in ChamP traps with bait packets and pheromone lures versus traps with bait packets alone. Fruit volume increased by four times from mid-June to mid-November. Olive fruit fly was found to oviposit on small olive fruit <1 cm³ shortly after fruit set, the maximum number of ovipositional sites per fruit occurred in October, and the greatest number of pupae and adults were reared from fruit collected in September and October. The highest numbers of pupae were collected from nonharvested fruit in March when high numbers of adults were captured in the same orchard.