The population structure and fecundity of the hermit crab Dardanus deformis were studied. Monthly samples were obtained between January and December 2003 at Costa do Sol, Maputo Bay, southern Mozambique. A total of 604 individuals was analysed of which 273 were males, 164 non-ovigerous females and 167 ovigerous females. The overall size–frequency distributions were unimodal, with males reaching larger sizes than both ovigerous and non-ovigerous females. Females were slightly more abundant than males, but the overall sex ratio did not differ significantly from 1:1. Dardanus deformis showed continuous reproduction throughout the study period. A small number of juveniles was obtained, suggesting that recruitment may take place in a different habitat than that occupied by the adults. The egg number increased with female size, and the high incidence of ovigerous females, may suggest a higher reproductive activity probably as a result of the rapid embryonic development in the study area.