A sectional dinic and epidemiological study was performed aiming to define the morbidity of the S. mansoni infection in the human population of Riachuelo, Sergipe, in Northeastern of Brazil. Eight hundred and fifty persons from the above community (25%) were random-selected and studied: 835 of them had a stool examination done by Lutz sedimentation (Hoffman, Pons and Janer) technique and S. mansoni egg counting by Katos' method. A skin test with antigen from adult worm was performed in 393 persons from the sample. The average prevalence of the S. mansoni infections in the population (viable eggs in the stool) was 50.54%. The prevalence of the infection by S. mansoni in children under 5 years was considerable lower (6.15%) than in the groups from 6-10 (41.5%) and in those from 11 to 50 years (average of 75% of infection). The skin test positivity was marked higher than the stool examination in ali ihese groups, but there was a good correaltion between these two methods in ali groups. However in the groups over 50 years there was a drop to 48.3% in the stool positivity for S. mansoni eggs but the skin test remained with a very high positivity (94.11%). There was no difference of positivity either in the stools or in the skin test in relation to the sex but regarding to the race there was a higher prevalence of the infection in blacks and ",mestiços" than in whites. Clinically 73.17% of the infected people were classified in group I (intestinal infection), 24.39% in group II (hepato-intestinal) and only 2.43% in the group III (hepatosplenic form). Biomphalaria glabrata was the single vector found in five focci surrounding the community. The infection rate by cercarias of S. mansoni in 1.208 snails colected was ofabout 1% (12 snails positives).