Aspects of the biology of the swimming crab, Portunus pe/agicus (Linnaeus, 1758). were investigated from trawl catches from offshore fishing grounds in Bahraini waters (designated areas A [northern], B [north-eastern] and C [southern], a total area of 1636 km2 ) and inshore grounds (traps) around Bahrain over 14 months. P. pe/agicus were found over a considerable range of temperatures (16-35 °C) and salinities (39-57%0). The abdomen of female crabs shows a distinct change in shape at about 7 cms carapace width (CW), when the pubertal moult takes place. In male crabs the shape of the abdomen remains unchanged throughout life. Breeding season lasts from March to November, with peck numbers of ovigerous females between June and September. Sex ratio analysis has shown that mature female crabs migrate from the shallow waters of areas A and C to the deeper water of area B. Since 73% of all ovigerous females caught were in area B, it was designated the major spawning ground. Females spawn at least twice throughout life and fecundity reaches as high as 1.54 million eggs brood-I. Larvae were found over a more restricted temperature (31-35 °C) and salinity range (40-48%0) than the adults. Certain regions of areas A, C and the whole of area D (west) have been identified as the major nursery grounds. Two recruitment periods were identified - a short period (June-July) and a much longer period (December-April). Relative abundance (crabs km-I trawled) varied spatially and seasonally being high during the summer months and low during winter through to spring. CW of crabs in areas A and B exhibits isometric growth with carapace length (CL). In area C, however, both dimensions indicated allometric growth with relatively shorter CL in the larger crabs. Male crabs were found to be relatively heavier than females at 2 cms CW, becoming more so above 6 cms CWo The growth rate for both genders ranged from 0.5 to 2.5 cms CW month-I. Base-line recommendations for the crab fishery management in Bahrain from this study include - minimum size limits, banning the capture of ovigerous crabs, banning fishing in the nursery areas as well as modifications to fishing gear.