To understand physician practices regarding the diagnosis of acute diarrheal diseases, we conducted a survey, in 1996, of 2839 physicians in Connecticut, Georgia, Minnesota, Oregon, and California. Bacterial stool culture was requested for samples from the last patient seen for acute diarrhea by 784 (44%; 95% confidence interval, 42%-46%) of 1783 physicians. Physicians were more likely to request a culture for persons with acquired immune deficiency syndrome, bloody stools, travel to a developing country, diarrhea for >3 days, intravenous rehydration, or fever. Substantial geographic and specialty differences in culture-request practices were observed. Twenty-eight percent of physicians did not know whether stool culture included testing for Escherichia coli O157:H7; 40% did not know whether Yersinia or Vibrio species were included. These variabilities suggest a need for clinical diagnostic guidelines for diarrhea. Many physicians could benefit from education to improve their knowledge about tests included in routine stool examinations.