Psychrotrophic bacteria were isolated from 227 pasteurized milk samples which had a shelf life in excess of 20 days at 7.2 C. Of 700 cultures isolated, 135 were resistant to heating at 72 C for 16 sec and were able to re-establish growth at 7.2 C. Thirty-five cultures, representing 15 different types were subjected to detailed examination to determine their actions on refrigerated milk, growth temperatures, thermal resistance at various temperatures, and their identities. The spore-forming genus Bacillus occured most frequently. The non-sporing types were assigned to the genera Arthrobacter , Microbacterium , Streptococcus , and Corynebacterium .