Moult cycle of Liocarcinus arcuatus in the Ría de Arousa (Galicia, north-west Spain) was analysed in relation to: (a) temporal changes; (b) spatial variability associated with both different environmental patterns (type of bottom, depth, etc.); (c) sex; and (d) body size. A total of 4275 females and 4103 males of L. arcuatus were caught in three different areas of the Ría de Arousa, selected as being representative of two types of habitat: a beach zone and two mussel raft culture zones (in the inner ría and in the outer area). Five moult stages were established on the basis of the degree of calcification in the exoskeleton. Only a small portion of the population (less than 15% in practically the whole year) were found monthly in one of the early postmoult stages (Stages A and B). Spatial and temporal variability of the moult cycle of L. arcuatus in the Ría de Arousa was found to be significant (P<0·001). There were no important spatial differences in the moult cycle of L. arcuatus females, by contrast, males showed a greater degree of spatial variability.