The work presented in this thesis consists of an investigation into the development of the female reproductive system of the human blood fluke Schistosoma mansoni. Worms were examined both under the light and electron microscopes, at all ages from 20 to 70 days post-infection. An investigation was also carried out into the various factors which affect the rate of development of the reproductive system. The development of the female system as a whole, as observed under the light microscope, is examined in Chapter 1, with a detailed examination of the ultrastructural development of each individual section of the system as observed using the electron microscope being presented in the succeeding chapters. Several factors were found to have an effect on the development of the female worms. These were the age of the worms themselves, the sex-ratio of the population as a whole and the population size. However, only the first two factors were found to effect the sexual development, the population size only affecting the overall size of the worms. The ultrastructure of the various components of the female reproductive system was observed at ages ranging from 20 to 70 days post-infection and the method of development of each component has been described as far as possible.