The olive fruit fly, Bactrocera oleae, is the main pest of the olive fruit and its expansion is exclusively restricted to the cultivation zone of the olive tree. Even though olive production has a century-old history in California, the olive fly was first detected in the Los Angeles area in 1998. Within 5 years of the first observation, the insect was reported from all olive cultivation areas of the state. Field-collected flies from five locations in California and another from Israel were analyzed on the basis of microsatellite polymorphisms in 10 microsatellite loci. These results were integrated with those of a previous study of olive fly populations around the European part of the Mediterranean basin. The analysis pointed to the eastern part of the Mediterranean as the putative source of the observed invasion. Moreover, samples from California were quite different from Mediterranean samples implying the participation of phenomena such as genetic drift during the invasion and expansion of the olive fly in California.