The initiation of schistosome infection by dermal penetration by cercariae, and their differentiation into schistosomula, provide the first interface of the host-parasite relationship. Overt, clinical consequences result from this interaction most often following combinations of species of schistosomes and hosts which are incompatible in regard to the development of mature, definitive host infections. The most common such situations result from human exposure to some of the schistosomes of waterfowl. It has been observed that these lesions, termed schistosome dermatitis or “swimmer's itch,” intensify upon repeated exposure to cercariae, and involve infiltrates of mixed cell populations. Severe dermatitis seldom follows the exposure of humans or other compatible definitive hosts to the cercariae of schistosome species which develop in man. However, numerous experimental studies have established that penetration by either Schistosoma mansoni, S. haematobium, or S. japonicum does not go wholly unnoticed within such hosts.