The sex ratio of adult worms has been observed biased towards males in Schistosoma mansoni under natural conditions. The origin of this bias is unknown. This paper determines whether males are more infective than females under controlled experimental bisexual conditions, and hence if the sex ratio is male-biased as a consequence of this. The male and female cercarial infectivities in uni- and bisexual vertebrate host infections using a range of controlled cercarial sex ratios were studied. The results showed that, in experimental unisexual infections, male cercariae were more infective than females, and that in experimental bisexual infections, male cercarial infectivity was similar to that of female, irrespective of cercarial sex ratio. Furthermore, cumulative male and female cercarial infectivity was maximal when sex ratio was equilibrated. The unbiased sex ratios obtained in our experimental bisexual infections are discussed in terms of behavioural and/or biochemical male female interaction. Alternative explanations of the natural biased sex ratio are proposed