Exposure of 472 animals to the cercariae of S. haematobium under conditions conducive to infection, and subsequent examination of these hosts has allowed an evaluation of the susceptibility of commonly used laboratory animals to infection by this parasite. 1. 1. All albino mice and hamsters exposed to cercariae became infected. Both are considered as usable laboratory hosts for experimental schistosomiasis with S. haematobium, although they display rather broad ranges in their susceptibilities. The former produces greater numbers of larger parasites but the latter may be the best all-around host, since the schistosomes reach the veins in the vicinity of the urinary bladder as well as those in the lower levels of the large intestine. Mice and hamsters could probably be used more effectively if exposed to larger numbers of parasites than employed in this study. 2. 2. Albino rats, cotton rats and guinea pigs are poor hosts for S. haematobium. The numbers of parasites are drastically reduced in early stages of infection in albino rats and guinea pigs. Rabbits and dogs are refractory to infection and cats produce almost negligible numbers of subnormal parasites although exposed to large numbers of cercariae. Goats can be infected but the return of schistosomes is extremely low. 3. 3. Monkeys and baboons are good hosts for experimental studies. However it must be realized that these primates as well as acceptable rodents demonstrate variable degrees of susceptibility or tolerance for S. haematobium. The parasite may or may not reach the vessels of the urogenital system which is supposedly a natural definitive location for S. haematobium in man. 4. 4. There is a slight predominance of male schistosomes if the overall return of parasites is considered. Only male schistosomes were recovered from guinea pigs. Female schistosomes were slightly in the majority when sex determinations were made using cercariae from naturally infected snails. 5. 5. Pathologically, the primates stand alone in demonstrating conditions of involvement comparable to those found in man. Pathologic changes in monkeys and baboons were inconsistent, the urinary bladder being involved in some individuals but not in others. In mice a few eggs were found in the tissues near the urinary bladder but no parasites were demonstrated. In hamsters there were both worms and eggs in the region of the urinary bladder, with some tissue damage. Albino rats and guinea pigs showed little tissue damage since eggs were deposited in neither the intestine nor the bladder. Cotton rats demonstrated only minor lesions since the parasites were few and only a limited quantity of eggs was found in the walls of the large intestine. A majority of albino mice and hamsters, and a few cotton rats possessed eggs in the liver. In this respect pathologic involvement was not remarkable. Eggs were not found in the livers of white rats and guinea pigs. 6. 6. Splenomegaly occurred in many of the mice infected with S. haematobium but only in two hamsters. There was no indication of splenic enlargement in albino rats, cotton rats or guinea pigs. The occurrence and the degree of splenomegaly in the primates was surprisingly low. There is no consistent relationship between parasite returns and splenic enlargement although this condition appeared more frequently in mice with prolonged periods of infection. 7. 7. Egg production and subsequent passage with the feces and urine were variable and seemed to depend mostly on individual compatibility or host-parasite relationship. Only moderate quantities of eggs were passed irregularly by mice and hamsters. The peak in egg passage with feces was reached at 16 to 24 weeks. Quantities of eggs were regularly recovered from feces after this period in some cases but in others egg passage ceased entirely. Cotton rats passed only limited numbers of viable eggs, whereas albino rats and guinea pigs passed none. Monkeys and baboons passed moderate quantities of eggs with the feces and on a few occasions in the urine. Their recovery from the primates was just as erratic as that recorded for rodents and when estimated on a per worm basis, the quantity of eggs produced is of the same magnitude.