Estimates of labour productivity loss due to schistosomiasis have been put as high as 40% but there are no satisfactory objective measurements of deterioration in physical working capacity to support these claims. The effect of s. mansoni infection on work performance in the Sudan has been investigated in the present study using the criteria of maximal aerobic power measured in the laboratory and the habitual activity pattern verified by energy expenditure measurements in the field. The study relates to economically active males aged between 18 and 45 years: infected and non-infected villagers, heavily infected canal-cleaners, hospital patients, physically trained soldiers and non-infected townspeople. All had received anti-malarial prophylaxis. Laboratory measurements included anthropometric, sociological, haematologic and biochemlcul investigations together with pulmonary function tests and bicycle ergometer measurements of aerobic work output using a semi-automated system. In 37 non-infected and 147 schistosomiasis-infected villagers no differences were found in pulmonary function, submaximal exercise performance and predicted maximum aerobic power. Statistically significant reductions in all these parameters were, however, found in a group of heavily infected canal cleaners. A changing pattern of quantitative egg excretion from moderate (<1000 eggs/g) to high (>2000 eggs/g) egg loads was shown to bo associated with decrements of up to 20% in maximum aerobic power. The results wore not attributable to anthropometric (particularly leg muscle volume which affects performance on the bicycle ergometer) or to nutritional differences between the groups, but predicted maximum aerobic power, egg load and haemoglobin concentrations were significantly correlated. In a further study of anti-schistosomal (hycanthone) treatment on two groups of villagers, one of which served as a control, a significant improvement in physiological work capacity occurred after treatment. The empirical findings presented in this thesis provide abasis for a clearer understanding of the relationship between schistosomiasis infection, tropical productivity and health of an infected community.