Previously unknown in Conus (Mollusca: Gastropoda) and poorly documented in the Australian region, imposex is demonstrated in 80% of females of six Conus species from Rottnest Island, Western Australia. In no case were the effects sufficiently severe to cause sterility, and reproduction appeared normal. Specimens from the same location preserved in 1975 were unaffected. With more than 500 species, Conus is the largest genus of marine molluscs. Because many species are conspicuous, widely distributed, and occur in populations whose past densities are known, they could serve as monitors of artificially induced pseudohermaphroditism and an early warning of future population responses to pollutants in marine environments at low latitudes.