Studies on the biology of Schistosoma margrebowiei include, a simple means of culturing and infecting Bulinus natalensis snails; the morphology and ultrastructure of various stages in the life-cycle; pathology; cercarial longevity and infectivity; cross-reactivity with S. mansoni rabbit anti-sera and the possible use of S. margrebowiei egg homogenate in the serodiagnosis of S. haematobium patients. A simple method of maintenance and infection of B. natalensis snails en masse, was found to yield a rapid and continuous supply of material. The results indicate that the size of snails at the time of exposure is an important factor in successful infection. A wide range of morphological and ultrastructural similarities were found between S. margrebowiei larval stages and those of other species of the genus. Whereas the adult worms are among the largest, the eggs, miracidia and cercariae of S. margrebowiei, are among the smallest in the genus. The pathology associated with S. margrebowiei, is due to deposition of large numbers of eggs in various organs of the infected animal. Eggs were not only recovered from the liver and intestine but following 50 days post-infection, from the spleen. A large number (10-15%) of the total eggs recovered from mice 45 to 65 days post-infection were deposited in the spleen. The cercariae of S. margrebowiei by utilizing their glycogen reserves, can live for up to 70 hours in fresh water at temperatures of 26-28°C. This observed life-span can be prolonged when water temperatures were decreased to $-2. °C. Cercariae kept in cold water although physically active and still infective, were found to be attenuated as measured by a reduced percentage of recovered worms compared with controls. The potential for immunizing mice with the hepatopancreas from infected and uninfected snails against schistosomiasis has been evaluated using S. mansoni. Although a reduction in the number of worms and eggs was observed in mice immunized with infected hepatopancreas when compared to the controls, this decrease was not significant. Sera from 53 patients infected with schistosomiasis were studied by ELISA using S. margrebowiei crude soluble egg antigen (SEA), S. mansoni SEA and cationic S. mansoni egg antigen (CEF6). It was found that S. margrebowiei SEA was more specific for the identification of S. haematobium infections.