This is the first record of intersexuality to have been found within the Cephalopoda. Seven out of 16 sexually mature Ancistrocheirus lesueurii males from southern African waters had nidamental glands in the mantle cavity in addition to a normally developed male reproductive system (spermatophoric complex, testis and hectocotylus). The frequency of occurrence suggests that feminization in male A. lesueurii is not a rare phenomenon in the south-eastern Atlantic. Normal A. lesueurii shows sexual size dimorphism, with females growing larger than males. The intersexual males formed a distinct size group intermediate between normal males and females, and their testis mass and spermatophore length were significantly larger than in normal males. The absence of oocytes and oviducts in intersexual males indicates that feminization represents pseudohermaphroditism. Intersexuality does not seem to affect male functionality and is apparently advantageous in that larger body size is accompanied by larger testis and spermatophores.