The population dynamics of the shore crab Pachygrapsus marmoratus was studied at two different rocky shores, Cabo Raso and Avencas, within the central Portuguese coast. The population structure at these shores, obtained by pooling monthly samples, revealed marked discrepancies. Unimodal, right-skewed size–frequency distributions at Avencas contrasts with the bimodal distributions obtained at Cabo Raso, corresponding to early recruits and larger adults. Recruitment timing and intensity together with habitat complexity are probable causes of the differences observed. At Cabo Raso an intense recruitment season was recorded from September to November, but very small juveniles were always present in samples. Monthly polymodal distributions are apparent but the size structure too complex to identify age groups through the sampling period. At Avencas, the influx of new juveniles is temporally restricted and year groups recognizable from main recruitment events taking place around October. Young-of-the-year crabs grow quickly to mature size and join the reproductive stock in the next breeding season from late March to early September in the study region. The growth pattern in both sexes follows a seasonal trend with minimum growth during the breeding months and maximum growth rates from late winter to early spring. Life span may reach four years, probably more in males. At Avencas, population maintenance seems to be highly dependent on favourable conditions promoting adequate settlement intensity and timing. Early recruitment is presumed to enhance the reproductive output of young-of-the-year individuals which comprise the bulk of breeding crabs. At Cabo Raso settlement rates are much higher and recruitment timing may be less critical since most breeding crabs belong to the 1-y+ class.