Pectinatus cerevisiophilus strains were subjected to mild hydrochloric acid extraction. Electron microscopic examination revealed that the bacterial cells still retained their original form. The extract contained vesicles, derived mostly from the outer membranes of the bacteria. The extracted material from ten isolates was studied by SDS polyacrylamide gel electrophoresis and a number of different protein patterns were found. These allowed two main groups and four subgroups to be distinguished among the isolates. The molecular weights of the major proteins varied between 13 and 62 kilodaltons. The mild hydrochloric acid extractable protein patterns provide a useful tool for typing different strains.