The populations of three sympatric hermit crabs, Pagurus excavatus, Anapagurus alboranensis and Anapagurus petiti were studied in a shallow (15–25 m) sandy area in the north-western Mediterranean. Seasonal abundance, seasonal size frequency, sex ratio and reproductive periods were examined. Pagurus excavatus was the largest and most abundant species in the zone. Pagarus excavatus and Anapagarus alboranensis are sexually dimorphic in relation to size, with males reaching larger sizes than females. However, the males and females of Anapagurus petiti reach similar large sizes. Anapagurus petiti has a sex ratio that is female biased, whereas A. alboranensis and P. excavatus are 1:1. Anapagurus petiti showed a higher reproductive activity during warmer seasons, while in A. alboranensis the proportion of ovigerous females was high throughout the year, with a minimum in autumn. The activity and clutch size of P. excavatus decreased during summer.