Preliminary observations on the biology of the freshwater cladoceran Monia irassa were carried out using the samples collected in the vicinity of Inchon City in Jun 20, 1992. A total of 208 specimens randomly sorted from the samples were measured for their body length(BL) and body width(BW). BL ranged 1.07-1.52mm(mean 1.29) and BW, 0.76-1.20(1.03). Embryonic development coincided with the increase of embryo length; stage I: sphere-shaped embryos just after being released from ovary(mean 0.26mm), II: embryos with deviloped embryonic appendages(0.39), III: embryos with well-developed antennae being identified by antennal spines(0.45). No peadogenetic reproduction was observed. Combined effects of embryonic growth and fecundity were positively correlated with BW( I : r^2=0.64, II: 0.42, III: 0.35, mean: 0.45), but not with BL. Fast increase in BW occurred in individuals with early embryos (stage I embryos: X coefficient of EL= 2.09) and expansion rate of brood pouch was gradually decreased with embryonic development (II: 0.67, III: 0.29). The number of embryos per batch ranged 4-25(mean 14.8) and more than 50% animals had 13-17 embryos. Decrease in the fecundity with embryonic development [stage I : 8-22 embryos (mean 16.1), II: 5-25 (15.7), III: 4-24 (14.5)] suggests the presence of prenatal mortality in the species.