Life history and population biology of paddle crabs, Ovalipes catharus were investigated at two South Island localities, Pegasus Bay and Tasman Bay between 1984 and 1986. In Pegasus Bay, crab distributions are related to moulting and breeding cycles. Males and females aggregate in sheltered bays and harbours during winter and mate there at the time of the female moult. After moulting, females move to spawning grounds which are probably offshore in deeper water whereas males move to open sandy beaches to feed. Growth rates of captive crabs kept at the Edward Percival Field Station, Kaikoura declined with increasing size as a result of lower moult frequency and percentage moult increment. Three recognizable phases of growth corresponded to juvenile, subadult and adult stages of the life cycle. Variation in individual moult increments resulted in increasing variation around mean instar size in successive juvenile instars. However, after sexual maturity was first attained, variation in instar sizes decreased. A simple population model based on the observed growth patterns of individuals, predicted that 13 instars would be discernible in a natural population. Thirteen post-larval instars were identified by size-frequency analysis of crabs from Pegasus and Tasman Bays. The interpretation of size-frequency data was supported by laboratory growth studies and tag-recapture data. During the first year of life, growth of O. catharus was influenced by the rate of accumulation of day-degrees above 5°C. Tasman Bay crabs grew faster than those from Pegasus Bay (on average 3°C cooler) but adults ultimately reached a similar maximum size. Maximum size (130 mm CW for males and 115 mm CW for females) was attained in Pegasus Bay in about 4 years and in Tasman Bay in 3-3.5 years. O. catharus is relatively long-lived and slow growing compared to other warm temperate and tropical portunids but is large and fast growing compared to other species of Ovalipes. In Pegasus Bay, females attained sexual maturity in the year of settlement at a mean size of 65 mm CW as shown by gonad development, evidence of copulation and changes in abdomen shape. The number of eggs per brood was related to female size and ranged between 1x105 and 1x106 eggs. Two or three broods are likely to be produced per year and up to 9 or 10 in a lifetime. In Tasman Bay, higher water temperature prolonged the breeding season and enhanced growth rates of crabs which in turn resulted in sexual maturity being attained sooner and at a smaller size (mean 50mm CW). Higher temperature and greater food availability also may have been responsible for enhanced survival of large crabs in Tasman Bay and a predicted increase in number of broods produced annually. The net result of differences in life history traits between the two populations was that reproductive potential of females in Tasman Bay may have been up to three times greater than that in Pegasus Bay.