Raw milk is identified with increasing numbers of outbreaks of gastroenteritis and is an important vehicle for transmission of Campylobacter infection. Unlike Campylobacter jejuni, Campylobacter fetus subsp fetus has not been associated with common-source outbreaks of gastroenteritis. This report describes an outbreak of gastroenteritis involving C jejuni and a thermotolerant strain of C fetus subsp fetus associated with raw milk. Fifteen (39%) of 38 persons who attended a banquet in Wisconsin in June 1982 developed acute gastroenteritis. Stool specimens were obtained from nine ill guests; four yielded C jejuni and three yielded C fetus subsp fetus. The C fetus subsp fetus isolates were identified fortuitously, in part because of unusual thermotolerance (growth at 42 degrees C), permitting isolation at temperature appropriate for C jejuni. Survey results implicated raw milk as the source of the outbreak. Findings provide evidence of a potentially emergent milkborne pathogen contributing to the risk of raw milk consumption and suggest that current diagnostic laboratory techniques may fail to identify significant foodborne agents.