Abstract: Biological studies of the mud crab, Scylla serrata (Forskal) collected from the Sundarbans mangrove ecosystem of Bangladesh were conducted during May, 2001 to February, 2002. The overall sex ratio of male to female was found to be 1:0.94. The sex ratio was uneven in most size groups and in most of the months. The modal class size was 81-90 mm Carapace Width (CW) in male and 71-80 mm in female. The monthly size-frequency distribution exhibited some variations. The relationship between carapace width and body weight were estimated as W = -128.25+4.06 CW, log W = -3.73+3.06 log CW or, W = 0.0078 CW3.06 for male and W = -89.59+2.387 CW, logW = -1.6035+1.8928 log CW or, W = 0.005 CW1.8928 for female. The condition factor of male crabs showed two peaks; first in 61-70 mm and second in 121-130 mm size group. In case of female it was highest in 61-70 mm size group. The relative condition factor was maximum at 125.5 mm CW and 65.5 mm CW in male and female, respectively. Gonadosomatic Index showed a single peak during May to July for female crabs.