The results of the trapping survey were analyzed to determine the taxonomy of various tree species infested by the olive fruit fly (Bactrocera oleae) in California. Since its first appearance in California in 1998, the olive fruit fly has spread from Los Angeles to 37 counties, including all of the state’s commercial olive growing areas. Olive fruit flies were trapped from 19 tree species belonging to nine genera distributed in seven families of angiosperms. Olives (Family Oleaceae) were the preferred host of the olive fruit fly. Family Rosaceae had nine host tree species followed by Rutaceae (five host tree species). Other host tree species were distributed in Anacardiaceae, Fabaceae (Leguminosae), Lythraceae and Malpigiaceae families. These hosts were mostly fruit trees with the exceptions of Brazilian pepper tree, carob, crape myrtle and ornamental plum. The host list reflects typical hosts and is not comprehensive. It is unknown if different olive cultivars are more attractive to the fly or more susceptible to fly damage. The pest directly attacks olive fruits and can devastate entire harvests. Adults feed on nectar, honeydew and other opportunistic sources of liquid or semi-liquid food. University of California scientists are now developing specific information about the olive fruit fly in California and have synthesized useful findings from Europe, where the pest has long been established.