A comprehensive study of the causes and treatment of acute “summer diarrhea” on the Fort Apache Reservation found that the peak incidence was in children under age two of whom two-fifths were affected at least once, and one-quarter of all children required hospitalization. Thirty-five per cent of rectal swab cultures yielded a recognized pathogen, primarily Shigella species. In spite of liberal antimicrobial usage on the reservation no Shigella isolate showed multiple drug resistance. Conversely, one-third of the isolates of enteropathogenic Escherichia coli serotypes were multiply resistant. S. flexneri predominated over S. sonnei by 4: 1. A significant correlation between poor home sanitation and the isolation of S. flexneri was found. The diarrheal attack rate rose simultaneously with rainfall, temperature and bacterial contamination of water sources. Diarrhea continues to be a serious burden to Apache people.