We studied the invasion of a California annual grassland by the shrub, Baccharis pilularis ssp. consanguinea (DC) C.B. Wolf. A series of aerial photographs indicated that the shrub had been present at the study site for 35-45 years but spread very little until the early 1970's. A period of slow expansion was followed by rapid outward spread of the shrub population and large increases in shrub density. Detailed demographic studies showed a period of rapid population increase from 1978 to 1983, with the highest stablishment rates in 1982-83. Establishment was strongly correlated with annual and spring rainfall amounts. An iniiall period of slow outward spread is thought to have profided foci for rapid spread during years of high rainfall.