During the summer of 1958 small numbers of California meadow voles immigrated to Brooks Island, San Francisco Bay, California. This report deals with subsequent patterns of dispersal and population growth in this previously vole-less environment from the inception of the population until the end of the second seasonal peak of population growth in May of 1960. The basic data for our analyses were derived from the study of runways intersected by line transects oriented across the major environmental gradient of the island. Brooks Island is a former hill-top isolated by the flooding of the valley of the Sacramento River which led to the formation of San Francisco Bay at the end of the Pleistocene (Louderback 1951). It is located on the east side of the bay approximately 800 yd south of the mainland at Point Richmond, Contra Costa County. The extent of suitable vole habitat is approximately 47 acres. The island is roughly triangular (P1. 4) with a high ridge (maximum elevation 163 ft) on the long north-south axis. There are now no regular human inhabitants on the island, but there are evidences of a ranch which was abandoned around 1900, and of quarrying operations which ended about 1940. The latter resulted in the formation of the bench on the west side and the large depression on the south-west end. The vegetation is a shrub-savannah type, ideally suited to Microtus californicus, and very similar to favoured habitat on the adjacent mainland. The predominant shrub is Baccharis pilularis. The grassland is rich in both perennials and annuals; species of Bromus, Festuca, Elymus and Lolium are prominent. Variations in habitat composition on the island correspond to differences in exposure and soil moisture. The west side is generally drier with more annual grasses and fewer shrubs. By contrast the east side is moister, more brushy, has more Rubus patches and generally more perennial grasses. Aridity is most extreme where mineral soil has been exposed by the quarrying operations. This general pattern of vegetation is clearly visible in Plate 4. The climate in the San Francisco Bay area is of the Mediterranean type; that is, it is characterized by moderate temperatures throughout the year, a winter wet season, and a summer dry season. This means that the voles ordinarily have green vegetation to eat from December into June. At other times the vegetation is dry and voles subsist mainly on seeds and roots. Because of its exposure to prevailing winds and afternoon sun, the west side has a longer and more severe dry season, while the soil moisture is higher and the growing season longer on the east side. This east-west gradient across the island is