The prevalence of malaria, schistosomiasis and onchocerciasis was determined in 1,106 residents of five villages in the Moyamba District, Southern Sierra Leone, to determine whether inland valley swamp (IVS) development was associated with changes in the prevalence of malaria, schistosomiasis and onchocerciasis in these villages. These parasitic diseases were studied in four villages receiving IVS, Food and Agricultural Organization (FAO) assistance and in one village not receiving FAO assistance. Malaria was the most prevalent infection, detected in 42.6% of the persons examined, followed by O. volvulus (17.7%), S. haematobium (0.6%) and S. mansoni (0.3%). Plasmodium falciparum accounted for 90.4% of the malaria infections, followed by P. malaria (2.1%), P. ovale (0.5%), and mixed infections (7.0%). The trend of infection to O. volvulus increased significantly with an increase in age. S. haematobium (0.6%) and S. mansoni (0.3%) infections were low and no Biomphalaria pfeifferi and Bulinus globosus were found in 33 IVS development swamps examined. These data indicate that IVS development is associated with an increase in the prevalence of malaria infection, but not in the prevalence of O. volvulus, S. haematobium and S. mansoni.