Diapause inMegachile rotundata(F.) is only partially facultative, since neither induction nor termination is under the exclusive control of environmental conditions. Induction is apparently controlled by an endogenous system, since the 1st offspring of spring generation females always contain the greatest number of emergent individuals. Although the diapause-terminating processes are favored by low temperatures, diapausing prepupae placed immediately into incubation will emerge slowly over an extended period of time. Emergent larvae and pupae are killed by prolonged storage at 1.7°C and by incubation at relative humidities below 50%. The optimum temperature for diapause development is approximately 10°C.