The relationship between the size of a given mating male Carcinus maenas (L.) (Brachyura, Portunidae) and the size of the female with which it was paired was studied for 1248 pairs of crabs collected from the shore In 764 of these pairs the female was in pre-moult and so the pair were in pre-copula. In the remaining 484 pairs the female had already moulted and the pairs were in copula. There were significant correlations between the sizes of the males and females in both pre-copula and copula pairs. It was found that male Carcinus collected in mating pairs and tested in the laboratory were unable to distinguish between females in terms of their size or stage of pre-moult. The positive correlation between the sizes of males and females in mating pairs on the shore is proposed to be, in part, a function of a mechanical constraint of the size of female that a given male can hold, defend and copulate with. In addition, encounters between solitary males and males carrying females, resulting in the formation of new pairs, appear to enhance the size-related mating pattern observed.