If food is in short supply, then provision of addition food should increase the density of Douglas squirrels. A squirrel population was supplied with extra food during the summers of 1977 and 1978 and winter of 1978-1979 in coastal coniferous forest at Maple Ridge, British Columbia. Food produced a 5-to 10-fold increase in squirrel density compared with control populations. Control densities generally varied from 3 to 10 squirrels per trapping area with the experimental population increasing to 65 animals during the winter feeding. This irruption was produced by immigration, more reproduction in females, and increased survival. After the food was withdrawn, the population declined to a level comparable with the controls. We conclude that Douglas squirrel populations fluctuate in accordance with the abundance of food. Territorial behavior may space individuals within populations of Tamiasciurus douglasii but to density levels determined by the available food supply.