Ten adult male volunteers (medical students) subsisted for seven days upon a chemically defined, low residue liquid diet, and consumed 1200-1800 calories per day. All stools were collected; three were cultured within the hour-a prediet stool, one collected on the seventh day, and a postdiet stool. Specimens were diluted anaerobically, and anaerobic cultures were streaked upon plates of prereduced agar media and incubated in Brewer jars. During the low residue diet, total fecal mass was relatively small and each subject passed only two or three stools. The mean reduction in daily fecal output was 70%. Mean counts of total aerobes were 10-7/gm throughout the study, and mean counts of total anaerobes were 10-10/gm. There was no overgrowth by opportunistic bacteria or fungi. The low residue food did not alter fecal flora; there was neither disappearance nor reduction of any bacterial group.