Quantitative zooplankton samples were collected twice a week during the period from April 1-July 30, 1994 in the vicinity of Inchon City. Moina irrasa occurred from April 24 to July 3 at temperature ranging between 18∼33℃ (optimum temperature: 25℃). This species always occurred together with other crustacean zooplankters. Its population density ranged from 150 to 18,900 individuals per liter of water and its proportion varied between 1.6 and 92.3% of zooplankton abundance. Swarm volume varied between 0.4 and 64 liter and the maximum size of the swarm of M. irrasa was 791,681 Individuals. Its body length (BL) and body width(BW) ranged between 0.98∼1.68 mm and 0.62∼1.52 mm, respectively and the fluctuation of BL coincided with that of BW. M. irrasa was clearly divided into two groups with its BW, viz., population with its BW> 1.1 mm and <1.1 mm. Mean fecundity in terms of the number of embryos per batch varied between 2.28∼27.91 and the maximum was 29 embyos. Intensive resting egg production occurred at the earlier period of appearance. No significant relationship was observed between brood size and adult size. Positive correlations of siginificance between BW of two populations (BW>1.1 mm: r^2= 0.32 and <1.1 mm: r^2=0.46), and brood volume, however, is observed respectively.