Detailed parasitologic, serologic, clinical and pathologic studies were conducted in chimpanzees exposed to varying numbers of cercariae of Schistosoma haematobium. Four animals were each exposed to a single dose of 500 or 2,000 cercariae and four others were each exposed monthly to 100 or 250 cercariae. Worm recoveries were 14 to 40% of cercarial exposures. No reduction in worm numbers occurred with infections of 7 to 17½ months' duration. Viable eggs were recovered both from feces and urine. Tissue digestion revealed egg-deposition patterns consistent with worm recoveries and pathologic lesions present. Intravenous pyelography, cystoscopy, and necropsy revealed pathologic changes similar to those reported for many heavily infected persons. Numerous inflammatory patches were found in the bladder mucosa and composite granulomata were scattered throughout all layers. The inflammatory patches of the ureteral mucosa, which resembled those of the bladder, caused ureteral displacement, ureterectasis, hydro-ureter, and hydronephrosis. The “bilharzial patch” appeared to be characteristic of the S. haematobium pathology of the chimpanzee, both in the urinary tract and in the rectosigmoid colon. A sequential evolution in bladder and colonic lesions from polypoid, to fibrous, to sandy patches was observed. There was a correlation between the parasitologic findings, serum biochemical test results, and pathological observations. Fluorescent-antibody tests and studies of passive cutaneous anaphylaxis detected the presence of antibodies, but the time-course development of the two antibodies was quite dissimilar. The chimpanzee was found to be well-suited for studies on the evolution of schistosomiasis hematobia.