Sex determination in mermithid nematodes is controlled by the environment. In this study, the relative roles that the genotype, mode of reproduction and postzygotic environmental influences have in determining the ultim.ate sex ratio of Romanomermts cul'icivoTax were examined. Sex ratio studies carried out under standardised conditions revealed that the initial number of parasites per host and the actual level of parasitaemia attained in each host are not related. At levels of 5 or more nematodes per host exclusively male nematodes were produced. For hosts with one parasite a high proportion of the nematodes differentiated as females but single male nematodes were found.