Populations of three indigenous species of leaf-cutting bees, Megachile centuncularis, M. leachella, and M. willughbiella, were compared to one of the alien bee, M. pacifica. In contrast to M. leachella and M. pacifica the two other species had so unfavourable a sex ratio that it proved prohibitive to propagation, while the rate of emergence showed the indigenous bees better adapted to our natural conditions than is M. pacifica. Parasitic wasps and bees were important enemies of the bees. Melittobia acasta occurred in all species reared in the greenhouse, Monodontomerus obscurus was present in the colony of Megachile willughbiella gathered in nature, and Pteromalus venustus seriously diminished the population of M. willughbiella when this bee was moved to a new nesting site. Coelioxys mandibularis is very common in naure, but was only observed once in connection with the studies presented. Several species of fungi were associated with the indigenous bees, while the alien M. pacifica was practically free of fungi. In M. centuncularis Ascosphaera proliperda and A. major caused chalkbrood-like diseases, though the latter seems merely a facultative parasite. A hitherto undescribed species of Microascus was associated to and probably pathogenic in M. willughbiella. The mucous ascopores of the fungi stick to the fur of the bees as they pass through one cell after the other when emerging. That is one way of transfer from one generation to the next.