In this study the influence of physical factors on the survival and infectivity of S. mansoni and S.haematobium miracidia was compared. The physical factors studied include termperature, light, gravity, depth, hydrostatic pressure, host-parasite dispersion, rate of flow of water and turbulence, ultra-violet radiation and the length of contact time. While the influence of one physical factor was being investigated all the others were kept constant and the levels of each factor applied were chosen in order to include the limits that may be encountered in the field. The criteria for the influence of a physical factor on miracidia were the infection rates produced in susceptible snails. In some cases the factor was applied to the miracidia alone for a period of time before exposing the snails to them. Miracidia were examined under a dissecting microscope for mortality or degree of activity of survivors, where the factor (such as temperature or ultra-violet radiation) affected their metabolic rates. The degree of activity of miracidia which indicated the extent to which a physical factor affected them was described as "ACTIVE", "SLOW" or "LETHARGIC" and was quantified by calculating the mean rates of movement of miracidia in those categories. For factors which acted as stimuli, such as light and gravity, the responses of the miracidia were recorded as a positive or a negative taxis. The use of carefully designed apparatus and simplified methods made it possible to reproduce the experiments several times, and statistical analysis of the results helped to assess the significance of differences obtained. These studies have shown that the survival and infectivity of S.mansoni and S.haematobium miracidia were influenced in a very similar manner by the different physical factors and that both parasites have a remarkable capacity to locate, select and infect their snail hosts. It was found that they were not limited by depth, hydrostatic pressure or dispersion and that they were capable of infecting snail hosts in water flowing at moderate rates. The different responses of S.mansoni and S.haematobium miracidia to light and gravity appear to increase their chances of meeting their particular snail hosts. The use of the physical factors studied to interfere with the infective capacity of schistosome miracidia and thereby reduce transmission in endemic areas has been discussed.