In Aug. 1974, 31 of 45 cases of Shigella sonnei infection in Dubuque, Iowa, USA, were traced to swimming in an 8-km stretch of the Mississippi River [USA]. Comparison of the first case in each affected family with neighborhood controls showed a significant correlation between swimming and illness (P < .0001). A significant association between diarrheal illness and swimming (P < .0001) was demonstrated by a retrospective survey of 60 families who camped at a park beside the river; the attack rate for swimmers who got water in their mouths while swimming was 18%. They swam in water where the mean fecal coliform count was 17,500 organisms/100 ml; the federal recommended upper limit for swimming water is 200/100 ml. A water sample obtained at the park swimming area 1 mo. after authorities banned swimming in the area yielded S. sonnei with the same antibiogram, colicin type and phage type as the isolates from 6 swimmers. Shigellosis.sbd.a disease that can be caused by the ingestion of only 10-100 organisms.sbd.can be contracted by swimming in polluted water.