Innvestigations on the morphology and reproduction of the freshwater cladoceran Daphnia similis were carried out using materials collected in waters adjacent to Inchon City in June 20, 1992. A total of 163 specimens randomly sorted from the sample was measured for their body length, viz., maximum body length(ML): 0.42-3.57mm, standard length(SL): 0.32-2.12 and body width(BW) : 0.21-1.70. Their means were 2.44, 1.39 and 1.16, respectively. Correlation analysis among three lengths revealed that the body shape of this species might hardely changed with the embryonic development in the brood space. The number of parthenogentic embryo varied from 1-27 per batch. Sexual females were less than 10%. Relationship between the number of embryos per brood and ML showed the saturation curve with the ML limit around 3-3.5mm. Fertility of D. similis was discussed in relation to body length.