Females of Crabro cribrellifer, nesting in a large aggregation in northern Michigan, were studied over three summers, 1981-1983. The majority of female wasps occupied and provisioned cells in single nests. Based on observations of individually marked females, a significant minority of the wasps (ca. 25%) provisioned in 2 or several nests. Hostilities between females interacting at nests were never observed. There is a significant diel correspondence between wasp provisioning activity and nest-entering activities of cleptoparasitic miltogrammine flies, Metopia campestris. These paparsites exact a heavy toll on the wasp population, with parasitism rates exceeding 25% in all years of this study. The discussion concerns ways by which parasite-host interactions may influence the nesting behavior of the wasps.