We carried out a comparative study of the dynamics of the sporocyst population and the growth rate of the host habitat (digestive and genital gland) within the Schistosoma mansoni/Biomphalaria glabrata system during monomiracidial infections. The volume of the digestive gland increased sharply during the first 2 weeks of infection and then decreased at 2-4 weeks post-infection during the exponential growth phase of the sporocyst infrapopulation. The effects of the parasite on the development of the host's genital gland resulted in a slowing of the growth of the ovotestis. These interactions, related to the demographic processes involved in the intramolluscal development of the parasite population, were interpreted in relation to the spatial and energy constraints imposed by the developmental needs of both the parasite and the host.