Published information regarding group size and composition, territories and reproductive success of the Acorn Woodpecker (Melanerpes formicivorus), a group-living bird, is limited. Scattered data concerning these characteristics have been found in a few references (Ritter 1938, Bent 1939, Skutch 1969). To collect more specific information about these aspects of Acorn Woodpecker ecology, I observed nine groups of these birds on the Stanford University campus, Stanford, California from 1973 through 1975.