This study reports the impact of infection by the mermithid nematode Gasteromermis sp. on the secondary sexual morphology and behaviour of its adult mayfly host Baetis bicaudatus. By applying the technique of flow cytometry I show that both male and female mayflies are infected by the parasite. The external secondary sexual characters of infected male hosts are feminized resulting in the formation of intersexes and it is possible that some infected males undergo complete sex reversal. The external morphology of infected female hosts is not altered. The behaviour of infected hosts is also affected. Infected mayflies join unparasitized females in upstream dispersal and ovipositing behaviour. I suggest that these alterations are induced by the parasite as an adaptive strategy that ensures: (i) upstream dispersal; (ii) return to water for completion of parasite life cycle; and (iii) maximum use of host nutrients.