The in vivo development of Schistosoma haematobium in the hamster was studied. Six stages of development were distinguished on the basis of morphological and histochemical criteria. Schistosomula reached the lung (stage 1) on day three post-infection, with maximum concentrations on day nine. Gut formation occurred in the second stage at day 18. In stage 3 'Organogeny' (day 24) males developed one testis and females a narrow uterus. Pairing and the development of males with sperm-containing testes and females with an ovary characterize stage 4 (day 28). In stage 5 (day 53) vitelline follicles begin to develop in the females. The final stage, oviposition, occurred on days 61-63 and was characterized by the appearance of fully developed eggs in the uterus.