Population characteristic of the mantis shrimp Lysiosquilta macullata Fabricius, were monitored on a reef flat at the Pari Island coral ecosystem, Seribu Island Jakarta, from May 1985 to April 1987 (two years). The population density fluctuated from 50 individuals ha^-1 to 475 individuals ha^-1. Small, medium and large individuals of Udang pengko were evenly spread over the reef flat area occupied by Udang pengko (L. maculata), there was no segregation by size. The occurrence of Udang pengko in terms of their distance from the reef edge, biomass and morphometrical characteristics was discussed. The statistical test for the differences between years showed the marked difference on their population density.