The morphology of the spermathecae (seminal receptacles) was studied in the grapsid crab Chasmagnathus granulata, both from the macroscopic and the microscopic points of view. Female adult crabs were collected throughout the year. Once in the laboratory, the right spermatheca was fixed in Bouin solution; histological sections were stained with haematoxilin–eosin. The observations indicated that the spermathecae belonged to the ‘ventral type’ described by Diesel (1991), related to the ‘hard female’ mating system of C. granulata described previously. They presented two distinct zones: the inner one (white-coloured), where the double layer oviduct opened ventrally; and the outer one, of translucent, ‘jelly-like’ appearance, where the communication with the genital duct could be observed. The spermatozoa were only observed in the ‘jelly-like’ zone, but not as discrete packets, suggesting that spermatozoa from different matings could mix in the spermathecae of C. granulata. The spermathecae also showed seasonal variation, with the lowest degree of development observed during autumn. Post-moulted females always showed collapsed spermathecae.