Sex ratios of adult schistosomes in mice are almost invariably different from 1·0 and are biased towards males. The bias applies to wild rats infected with Schistosoma japonicum and trapped in an endemic area of the Philippines (male: female ratio = 1·7). It also applies to cercariae of snails collected in such areas and assessed by infection of laboratory mice using cercariae from individual snails (male: female ratio may approach 6·0). Experiments were designed to determine if duration of infection in the mammalian host was a factor that influenced the sex ratio of miracidia used for infecting snails and subsequently mice. BALB/c and C57BL/6 mice were infected with 100 cercariae of S. mansoni, and liver eggs harvested at early and late time points for infection of snails and production of cercariae. Two phenomena were demonstrated: firstly, a more pronounced male bias when eggs were harvested late compared with early in infection; secondly, a reduced apparent hatchability of eggs in BALB/c compared with C57BL/6 livers. The possibility is raised by the data that female miracidia within eggs of chronically infected individuals may be more prone to immune damage than male miracidia with important epidemiological consequences.