The benthic macroinvertebrate community structure and its seasonal fructuation in the Naktong estuary was studied during the period of August 1984 to April 1985. The benthic macroinvertebrates on each season were collected at total 12 sites over the Naktong estuary from the upper seawater reached region to the lower mouth of the estuary. The upper part of the Naktong estuary including Mulgum (St. 1) and Kup`o (St. 2) comes under the nearly freshwater region and the mouth of the estuary including Shimnidung (St. 6-1), Changjado (St.6-2), Daemadung (St. 6-3) and Chinudo (St. 6-4) comes under the seawater region. The other sites are covered brackish region. Especially Chomanp`o (St. 7), located at the branch river of Naktong estuary, comes under the real freshwater region on account of the sluice of Chugnim River. Quantitative data were investigated through the analysis of individual number (No./㎡), dominance indices (DI), species diversity indices (H`) and so on. The results are as follows: 1. The total benthic macroinvertebrate taxa collected from the Naktong estuary during Aug. 1984-Apr. 1985 composed of 61 species, 53 genera, 42 families, 19 orders, 7 classes and 3 phyla. 2. The species occurance at each site showed the most abundant at the sites of seawaters (Chinudo (St. 6-4) ) and real freshwaters (Chomanp`o (St. 7)), and decreased at the sites of brackishwaters (Kup`o (St. 2), Maekdori (St. 3-1) and Hakchang-dong (St. 3-2)). 3. The total individual number according to each season showed poorest in winter season, and increased in order of spring, summer and autumn. 4. The total individual number according to the studied sites showed more abundant at the sites of seawaters and real freshwaters than at the sites of brackishwaters. 5. The total individual number according to the main taxa showed most abundant in Crustacea, and decreased in order of Polychaeta, Gastropoda, Pelecypoda, Insecta, Oligochaeta and Hirudinea. 6. The total biomass (wet weight) showed a little seasonal, fluctuation and showed more or less abundant at the sites of seawaters. According to the main taxa the most abundant in Pelecypoda, and decreased in order of Crustacea, Polychaeta, Gastropoda, Oligochaeta and Hirudinea. 7. The dominant species are different according to the seasons and sites. Generally Neanthes japonica, Tylorrhynchus heterochaetus, Corbicula japonica, Ilyoplax pusilla, Scopimera globosa, Gammarus sp. are abundant. 8. The dominance indices (DI) showed 0.36∼1.00, and generally more or less low at the sites of seawaters and real freshwaters. 9. The species diversity indices (H`) showed 0.73∼2.55, and generally more or less high at the sites of seawaters and real freshwaters.