A randomized, double-blind, placebo-controlled field trial of a topical antipenetrant lotion, 1% niclosamide, applied daily to the upper and lower limbs of farmers occupationally exposed to Schistosoma mansoni cercarial- infested water, was conducted in the Nile Delta to assess its safety and efficacy in preventing reinfection. Farmers aged 18-40 years were treated to cure their S. mansoni infections three months prior to the onset of the trial. Subjects were randomly assigned to receive niclosamide or placebo lotion that was self-applied daily for five months. A total of 186 subjects met the inclusion criteria and completed the trial. The exposure to schistosomal-infested water occurred during routine irrigation activities from June to November 1991. Stool specimens were evaluated monthly during and for two months following the lotion application period. The subjects applying the niclosamide lotion were comparable to those applying placebo lotion in age (mean 30 years for both), total water contact (184.5 hr versus 173.8 hr), reported lotion application compliance (88% versus 92%), and reported water contact involving skin exposure other than upper and lower limbs (23% versus 27%). The schistosomal reinfection rate was lower in the niclosamide group (53.3%) compared with the placebo lotion group (71.3%), (P < 0.02). Increased protection might be obtained with total body application for shorter, less intense, water contact exposures.