We studied the spread of the newly introduced parasitic mite, VarmajacobsoniOudemans, within California's population of the feral honey bee,Apis mellifera L., by examining worker bees taken from 208 colonies in 1990, 124 of which were examined again in 1993. The samples taken in 1990 did not containV. jacobsonimites. In 1993, 75% of the colonies examined in an area located near Sacramento, CA, no longer existed, and all surviving colonies were infested withV. jacobsoni. In an area located near the Californian central coast, 84% of the nest sites examined were occupied and few colonies contained detectable levels ofV. jacobsoni. The probability of survival for colonies that have not survived a winter yet (founder colonies) has been previously reported to be low in areas with a temperate climate. Data collected in an area with a low level ofVarroamite infestation suggest that the probability of founder colony survival is higher in California with a mostly Mediterranean climate. The data collected in areas with a high level of Varroa mite infestation suggest that the parasite reduces the mean life span of feral honey bee colonies in California to go between 6 mo and 1 yr. The parasite was widely spread in areas with a high density of commercial colonies. This suggests that the fast spread of the parasite is caused mostly by migration of commercial colonies. We discuss the consequences of the decline of the feral honey bee population on pollination and on the invasion of California by Africanized bees.