The preferred host of the cluster fly, Pollenia rudis Fab., in Ontario is reported to be the earthworm Eisenia rosea Sav. Periodic field digging revealed the immature stages of P. rudis feeding, or in close association with, E. rosea. In addition P. rudis was reared in the laboratory on E. rosea but could not be reared on any of the other species of earthworm found in the field.For the first time in North America the complete life history, along with each of the immature stages of P. rudis, is described. The differences between P. rudis from North America and those from France are indicated.Finally, the authors speculate that there are two strains, or species, of P. rudis in France: one is host-specific to the earthworm Allolobophora chlorotica Sav. and the other to E. rosea. Only the strain with E. rosea as its preferred host has as yet been reported from Canada.