To determine some of the influences on nesting densities of bumble bees I examined the dispersion and occupancy of 35 nests in an old field on Amherst Island, Ontario, Canada and 39 nests found by Cumber (1953) in England. Densities up to 1 nest per 200 m2 were observed. Overall, Cumber's nests were randomly dispersed, but nests on the ground surface were aggregated. Nests of surface-nesting species were consequently more aggregated than expected at random, whereas surface-nesting and underground-nesting species seldom nested near one another. This distribution of nests probably reflects the nesting habits of the rodents that originally constructed the nests, rather than interactions between bees. On Amherst Island, abnormally wet spring weather apparently hampered normal colony development, particularly of underground-nesting bees based on a low frequency of underground nests, scarcity of workers of two underground-nesting species compared with the relative abundance of spring queens, and delayed appearance of workers of all species relative to the previous year. The nests of two common species were randomly dispersed, but those of a third species were regularly distributed, suggesting that intraspecific interactions may limit nesting density for this species.