Diel, lunar, and seasonal abundance, as well as population structure, of the free-swimming stage of the pea crab Pinnotheres bidentatus Sakai, 1939, from tidal waters off the west coast of central Taiwan were examined. A total 5,123 specimens were collected from tidal waters of the Kaomei coastal flat. These crabs were found year-round but mostly during fall to early winter (October to December 1994 and September 1995), with a peak in early December 1994. Numbers of crabs dropped sharply in mid-December 1994 and remained low during the months of January to April 1995, with slightly increasing numbers during mid-May to September of the same year. Daily abundance of the crab appeared to be related to new and full moon phases, indicating a semilunar pattern of swimming activity. Moreover, pea crabs were only collected during nighttime flooding tides, suggesting a nocturnal swimming activity which may have advantages for avoiding fish predators. Size structure of the crab indicates that P. bidentatus has life-history characteristics of which males die in the hard stage and females subsequently lose their swimming hairs and become commensal with their host. The overall sex ratio was 1:1.3 (2,206 males and 2,917 females) and varied according to month and size. The continuous occurrence of small individuals (1-mm size class) year-round indicates a long period of recruitment which may be a consequence of the long reproductive period of P. bidentatus.