We report on the occurrence of Sarcophaga megafilosia and S. meiofilosia, two flies that are parasitoids of littorinid snails, along the east coast of Australia from Cooya, Queensland (16°27'S) to Settlement Point, New South Wales (31 °24/S). Shells of attacked snails containing larvae and pupae of these parasitoids remain attached to the substratum by a dark ring of dried mucus that forms around the aperture, so the leaves and branches of mangroves, Avicennia marina, were searched for such shells, and flies that subsequently emerged were identified. The majority of snails attacked by S. megafilosia and S. meiofilosia were Littoraria filosa, but both flies emerged from L. luteola, and S. megafilosia also emerged from L. philippiana. The two parasitoids were found at the most northern site sampled and extended south to approximately 27°S, beyond which L. filosa became uncommon. Littoraria filosa is strikingly polymorphic for shell colour. Since S. megajilosia selects for crypsis in L. filosa and was found at most sites sampled, we suggest it is likely to be important in maintaining this polymorphism. We also suggest the wider distribution of these parasitoids may be similar to that of L. filosa and thus extend both westwards and northwards into the Indo-Pacific below the equator, and that other sarcophagid parasitoids of littorinid snails may be found in the central Indo-Pacific.