Zonal distribution and population biology of Ilyoplax frater were studied in a mangrove mudflat area of Pakistan.The crabs were collected from Korangi creek through transect and quadrat method from low tide level to high tide level. Twotransects were delimited in a mangrove area of Korangi creek (24°79'N/ 67°20'E). On each transect, three 0.25 m quadrats weresampled at three tidal levels on a monthly basis during low tide periods from March 2001 to February 2002. A total of 1124 crabswere obtained, of which 482 were males and 642 were females. Density of crabs varied between 0 and 90 /m'. The density andsize distribution varied and showed significant differences from low to high tide level, and were positively correlated with thepercent moisture, percent organic matter and sediment grain size. The carapace width (CW) ranged from 2.5 to 11.5 mm for maleand 2.5 to 11.0 mm for female and was not significantly different. The overall sex ratio did not differ significantly from the expected1:1 throughout the year in small crabs but was significantly different in adult crabs (x^2= 49.73) with more male crabs. Sizefrequency distribution showed recruitment of juvenile crabs (< 4 mm) nearly throughout the year except during June and July.Presence of ovigerous females in all months with seasonal peaks in September, October, December and May indicates seasonalcontinuous breeding. Weight of egg mass increases with weight of ovigerous females and show positive linear relationship. Theestimated mean diameter of egg was 2.83±0.25μm, and the average number of eggs was
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