Imposex is characterized by the development of male characteristics, such as the penis and vas deferens, in female prosobranch gastropods, caused by organotin compounds contamination. In Brazil, imposex was observed in the muricids Stramonita haemastoma and Stramonita rustica and in the olivid Olivancillaria vesica. During the low tide, 54 adult individuals of Leucozonia nassa were manually collected in February 2004 at Ilha do Japonês tidal flat, Cabo Frio County, Rio de Janeiro State, south-east Brazil. About 98.15% of females presented imposex (N = 30). Their penises were smaller than male ones. Only 1 imposexed female presented a biphallia problem, showing 2 flattened penises with a small scourge in the edge. This is the first record of biphallia in an imposexed female of fasciolarid Leucozonia nassa.
