Rapana venosa (Valenciennes, 1846; Gastropoda, Muricidae), 'Rapa whelk', is a predator of bivalves, native to the Japanese seas. It has been reported in the Northern Adriatic Sea since 1973. Recently, its biogeographical distribution has been widening (probably favoured by ship traffic) including the Atlantic coasts of the USA, Argentina and France, where the species colonised transition zones, such as estuaries or lagoons, economically important for shellfish harvesting. This work investigates the population structure of the Rapa whelk (size classes distribution, sex ratio) in Cesenatico (Emilia-Romagna coast), where local fishermen have been recently observing increasing numbers of Rapana in their by-catches. During summer 2001, analyses were performed on sexually mature adult specimens obtained from sandy bottoms and artificial rocky breakwaters. Male and female individuals were reported in approximately equal numbers with a minimum shell length of 67.0 and a maximum of 136.7 mm. Rapana collected on breakwaters were significantly larger and heavier than Rapana from sand substratum (ANOVA, P < 0.05). Sand shells and rock (breakwater) shells differed also in colour and epibiont cover. The role of man-made hard structures such as breakwaters in maintaining and promoting a further expansion of R. venosa is discussed.