The ash whitefly,Siphoninus phillyreae(Haliday) (Homoptera: Aleyrodidae), invaded and rapidly spread throughout California beginning sometime before August 18, 1988 when it was first detected. The ash whitefly's previous distribution was limited to Europe, the Middle East, and north Africa, where it feeds primarily on trees and woody shrubs. In 1989, outbreak populations of the ash whitefly occurred in several major urban centers in California, defoliating ornamental trees commonly planted by local governments and homeowners. Outdoor activities by homeowners also were curtailed by the high number of adult whiteflies in the air that posed a respiratory health threat. Within 3 years of the whitefly's invasion of California,Encarsia inaron(Walker) (Hymenoptera: Aphelinidae), was imported, mass reared, and released in 43 of 46 affected counties. The summer infestation density of the ash whitefly before releases ofE. inaronaveraged 8 to 21 individuals/cm2leaf. Within 2 years ofE. inaronreleases, the infestation density of the ash whitefly averaged 0.32 to 2.18 individuals/cm2leaf. The decrease in the ash whitefly density resulted from the rapid spread and establishment ofE. inaron.Parasitized ash whitefly, 1 year afterE. inaronreleases, averaged between 63 and 97% throughout the summer. Based on the economic value of preserving healthy ornamental/evergreen pear and ash trees, the ash whitefly biocontrol effort provided $219,822,823 and $298,803,970 in esthetic benefits to California in wholesale and retail replacement values, respectively. For every dollar spent by the State Biological Control Program and the University of California, approximately $181 in wholesale and $245 in retail esthetic value for the primary hosts of the ash whitefly were preserved.