Aspects of the growth and reproductive biology of the grapsid brachyuran Hemiqrapsus crenulatus (H. Milne Edwards 1837) have been studied in the Avon-Heathcote Estuary (43° 33'S, 172° 44'E), Christchurch, New Zealand. Relative growth analysis revealed that males became mature between 13.0 and 15.0mm CW whilst females achieved maturity between 9.0 and 11.0mm CW which coincided with the size of the smallest ovigerous females found in the field. Differences in the allometric growth of secondary sexual characters were also apparent. Monthly size frequency distributions over a two year sampling period revealed that the population consisted mainly of crabs in the middle size classes (12.0mm to 20.0mm CW) which maintained a 1:1 sex ratio however, males dominated all other size classes. The breeding season, based on the occurrence of ovigerous females, spanned 8-9 months from June to January/February. This extended breeding season could be attributed to some females producing successive broods with or without an intervening moult. Females were capable of transmoult sperm retention and ovarian and embryological development was cyclic. Laboratory studies indicated that a minimum incubation time of 45 days was required although lower salinities increased incubation period and egg size. Attempts at rearing H.crenulatus zoeae in various combinations of temperature and salinity were unsuccessful due to their apparent inability to feed. Absolute growth analysis indicated that males have larger growth increments and reached maturity in a year in their 11th instar, whereas females became mature in their 9th instar after only 290 days. A seasonal incidence of moulting was apparent with observed peaks occurring before and after the breeding season.