Pasteurella haemolytica was isolated from three of 18 grass samples and four of 18 water samples collected from two grazing fields occupied by sheep. This microorganism was also isolated from three of nine straw bedding samples collected from a pen housing ewes affected by mastitis caused by P. haemolytica. The same ewes developed scabbed papilloma-like lesions on the teat and udder skin. These lesions were colonized by P. haemolytica of various serotypes. Colder, wetter weather seems to prolong the survival of P. haemolytica in the environment of sheep, Survival of virulent strains of P. haemolytica in the environment could accumulatively increase the bacterial count, contributing to their transmission from animal to animal. The preference of P. haemolytica for colder, wetter conditions was confirmed in the laboratory where this microorganism survived longer in distilled water, phosphate-buffered saline, Todd-Hewitt broth, and ewe's milk kept at 4 degrees C.