Cross-section studies on prevalence and morbidity of schistosomiasis mansoni are presented for the State of Rio Grande do Norte, as part of a long term project which is being carried out in Northeastern Brazil. The survey was made in 44 localities, including rural and urban areas and covering the Maxaranguape irrigation valley. Prevalence of schistosomiasis and other helminthic infections varied a great deal. Percentage of hepato-splenic forms of schistosomiasis ranged from 0 to 4