The variation in the development of the reproductive system of female Schistosoma mansoni from single six infections is described. Vitelline cell development was assessed by staining for phenolic substances and the development of Mehlis' gland and the ovary by electron microscopy. Although there was considerable variation in the development of worms for individual mice harbouring infections of the same age, the men values indicated that there was a low level of differentiation and turnover of vitelline cells in worms 30-200 days post-infection. The proportion of females with possessed mature vitelline cells showed a general increase with age. The ovary was always in an immature stage but the development of Mehlis' gland showed some variation. The results suggest that the stimulus given by the male to the female is not necessarily to initiate development of the reproductive system but to increase the rate of development of the vitelline gland and to co-ordinate the development of the entire reproductive system.