a cross-sectional study on the prevalence of schistosomiasis mansoni in three sites of the "baixada ocidental maranhense" was carried out in 1993 in: alegre (in the municipality of s？o bento), alian？a (in cururupu) and coroatá ii (in the municipality of s？o jo？o batista). results were compared to those of another study performed at the same sites and in similar conditions, in 1987. the entire population of the three sites, with few exceptions, was submitted to fecal tests using the kato-katz method and immediate intradermal tests for schistosomiasis in both studies. subjects with positive results in one of these tests were clinically evaluated by a physical examination. in 1993, the total of 827 subjects were submitted to fecal examination and 826 to intradermal test. schistosoma mansoni eggs were found in the feces of 154 (18.6%) subjects, while 478 (57.9%) subjects presented a positive intradermal test. stool examination was carried out in 367 subjects in alegre with a positivity rate of 14.9%; the intradermal test, performed in 366 subjects, was positive in 47.5% of the cases. in alian？a, 277 subjects had their feces examined and were submitted to an intradermal test, with a positivity rate of 34.4% and 70.7%, respectively. finally in coroatá ii, 183 inhabitants submitted to fecal and intradermal tests had positivity rates of 2.2% and 59.0%, respectively. when the present data were compared to those obtained in the survey performed in 1987, a significant decrease in the prevalence of infection by s. mansoni was observed in alegre and coroatá ii, and a prevalence increase in alian？a.