Ten cases (including three laboratory confirmed cases) of E. coli O157:H7 occurred in a small California community. These illnesses were traced to the consumption of hamburger that had been purchased from a local grocery store. E. coli O157:H7, phage type 31, was isolated from the meat leftover from one person's home. This was the same strain that had been isolated from the laboratory confirmed cases. E. coli O157:H7, phage type 31, was identified in additional ground beef samples taken from one market. E. coli O157:H7 was identified in a ground beef sample from another market, but it was phage type 4 which was not associated with this outbreak.