Schistosoma mansoni is sometimes encountered in the United States because it is endemic in Puerto Rico. Routine stools for parasite ova are often not sufficient, because the adult lays its eggs in the venous system, and the ova must penetrate the intestinal mucosa to appear in the stool. In difficult cases, proctoscopic rectal biopsy with a fresh unstained crush preparation of the biopsy specimen has been advocated. Serologic tests have been developed; the tests most commonly mentioned in the literature are complement fixation (CF) and immunofluorescence. There is some disagreement on how much help the tests can provide. The major problem is differentiation between past exposure and current active infection.