A multistate outbreak of hepatitis A was traced to a campground in Louisiana. Among 822 campers during one weekend, 20 developed hepatitis A. Case-patients ranged in age from 4 to 36 years; the highest attack rate (6.4%) was for children aged 5-9 years. A case-control study revealed that case-patients were more likely than controls to have swum in a public swimming pool on Saturday afternoon (19/19 vs. 26/38; odds ratio [OR], undefined; lower 95% confidence limit, 1.7). Case-patients were more likely than controls to have swum in the jacuzzi pool (16/19 vs. 10/26; OR, 8.0; 95% confidence interval, 1.5-47.1) or adult pool A (19/19 vs. 15/26; OR, undefined; lower 95% confidence limit, 2.6). Case-patients were also more likely to have swum for greater than 1 h and to have put their heads under the water. Because of the design of the filtering system of adult pool A, a cross-connection between a sewage line and the pool water intake line was possible. This outbreak may have been caused by transmission of hepatitis A through swimming; thus, swimming may serve as a mode of transmission of hepatitis A virus, especially among small children.