Physiological tests of work performance and measurements of field productivity were made in 194 Sudanese cane cutters in order to study the effect of Schistosoma mansoni infection. The cane cutters were selected from two age ranges (16–24 and 25–45 years) and subdivided into three clinical groups: not infected, infected with, and infected without clinical signs of hepatosplenomegaly. Men infected with Schistosoma haematobium, malaria (blood film), or with hemoglobin levels less than 10 g/100 ml were excluded. There was a statistically significant (P < 0.002) higher mean hemoglobin concentration in those not infected, but the mean difference was less than 1 g/100 ml. Submaximal responses to exercise on a stationary bicycle ergometer, oxygen intake, ventilation, tidal volume, cardiac frequency and estimated maximal aerobic power output calculated both in absolute terms and relative to lean body mass and leg volume were similar in the six groups of cane cutters. No significant differences were found in physique, body composition or in thermoregulatory function tests. The cane cutters were found to have little natural acclimatization to heat in terms of sweating capacity when compared with a group of fully acclimatized Sudanese soldiers. The mean productivity (mean daily weight of cane cut per man) was significantly correlated with the individual's estimated maximum aerobic capacity determined in the laboratory, but not with the degree of S. mansoni infection. The noninfected group was less “efficient” (mean productivity:oxygen intake) during cutting than the infected groups but a larger proportion of the noninfected were in their first season of cutting. There was a positive correlation between the number of seasons' cutting experience and the individual's age, degree of infection and mean productivity. Cane cutters studied in this investigation were a relatively fit, active population from whom the more seriously ill were excluded. These results do not, therefore, necessarily reflect the effects of S. mansoni on physiological work capacity and productivity of more static populations in areas of high endemicity.