Thirty-three of 34 persons exposed for the first time to cercariae of Trichobilharzia stagnicalae, the chief cause of schistosome dermatitis in the United States, had only mild, innocuous and inconspicuous lesions. However, when 27 of those persons were given from 2 to 8 additional exposures to the same species, the reactions in most cases became progressively stronger. and 21 eventually had typical schistosome dermatitis or reactions approaching it in severity. These lesions were induced by relatively small numbers of cercariae and, in some cases, occurred within a very short time following the first exposure. Reactivation of earlier lesions under the influence of a subsequent reaction was observed in several cases. A generalized skin reaction was noted in conjunction with experimental schistosome dermatitis in several instances. It is concluded that the reaction in the skin of man to cercariae of nonhuman schistosomes is essentially a sensitization phenomenon. Evidence from 3 cases suggests that the sensitivity may persist for many years. Interpretation of the reaction as a sensitization phenomenon in which repeated exposures may be required before the typical severe reaction is elicited will explain some of the reported variations in the severity of the schistosome dermatitis reaction.