We examined effects of food and den site supplementation on population dynamics of Glaucomys sabrinus and Tamiasciurus douglasii in mature second-growth forests in British Columbia, Canada. We tested the hypothesis that populations of these squirrels were limited by abundance of food, not den sites. Populations were sampled intensively from June 1996 to March 1999 on controls and grids supplemented with food, food and nest boxes, and nest boxes. Analysis of variance revealed no differences in movement, abundance, recruitment, body mass of males, and percentage of males breeding among treatments for G. sabrinus or T. douglasii. Survival of G. sabrinus was not significantly different during pretreatment, but was significantly higher on grids supplemented with food during posttreatment. Occupancy rate of nest boxes in stands supplemented with nest boxes and food was 6- to 12-fold higher than in stands supplemented with nest boxes only. G. sabrinus occupied the majority of the nest boxes. We concluded that G. sabrinus used nest boxes readily but their populations were not limited by availability of den sites; availability of food appeared to have a significant effect on their populations. T. douglasii was not limited by availability of food or den sites during our study.