An outbreak of dermatitis occurred among county jail inmates who removed dead fish from the Sacramento River in California after a spill of metam sodium. The spilled chemical decomposes to methylisothiocyanate (MITC), a known skin irritant. A retrospective cohort study was conducted among the inmates and their crew leaders. Among 42 jail group members, 27 had dermatitis involving the feet and ankles; dermatitis was associated with lower extremity water contact (RR = 3.4, 95% CI 1.0-11.8); the attack rate increased with length of time spent in the water. For comparison, other state and federal employees who worked in the river at the same time were also interviewed. None reported dermatitis. Over three-quarters (24/31) of these other clean-up workers whose feet became wet changed to dry clothing immediately; none of the jail group changed immediately. The river concentration of MITC measured 20-40 ppb at the time of exposure. We speculate that prolonged wetness, occlusive boots, friction, and heat contributed to chemical irritation at this low concentration; the experience of the other clean-up workers suggests that this outbreak could have been prevented.