The over-all uncorrected prevalence rates of bilharziasis determined in this survey were-Control Division, 59.5%; Rural and Reclamation Divisions, 35.9%; Urban Division, 21.0%. There are significant differences in rates of infection between sections within a division, between adjacent villages and even between different parts of one village. Prevalence increases rapidly with age up to about the age of 14 years, declines somewhat up to the age of 40 years and then remains fairly constant at a rate of about 30%; the age-group 0-8 years should provide the most sensitive group for the assessment of control measures. S. mansoni infection is acquired more slowly than S. haematobium infection during childhood and is more persistent among adults.Except for the youngest age-group, bilharziasis rates are higher in males than in females, but more detailed analysis shows that this is true only for farmers and farm labourers and for those who swim. In respect of occupational categories, farmers and farm labourers, with prevalence rates of 50.6% and 41.6%, respectivelx, bear the brunt of the infection, since they constitute 48% of the population, although fishermen (60.4%) and boatmen (52.0%) have higher infection rates.Differences in bilharziasis rates can also be related to differences in religion, educational attainment and domestic habits (swimming, washing clothes, utensils and cattle) according to the opportunity provided for contact with polluted water. Swimming, because of the thorough exposure to possible schistosome infection that it provides, is one of the most important activities involved in the transmission of bilharziasis.