Biodiversity and seasonal community composition of benthic macroinvertebrates were studied from Upo wetlands in Gyeongsangnam-do, Korea, comprising Upo (4 sites), Mokpo (2 sites), Sajipo (1 site), Jjokjibeol (1 site), Yeobeol (1 site), and Topyeongcheon (2 sites) areas from October 2002 to August 2003. As a result, it was known that Upo wetlands retained relatively well-preserved littoral zones which may provide good habitats for benthic macroinvertebrates; however, frequent disturbances of littoral zones caused by flood were the major factor affecting on the survival and distribution of benthic macroinvertebrates in the areas. During the study period, a total of 135 species of benthic macroinvertebrates in 10 genera, 59 families, 16 orders, 7 classes, and 3 phyla were collected those of which are the highest degree of diversity of the taxa ever known in Korean wetlands: aquatic insects 103 spp. (Diptera 27 spp., Odonata 24 spp., Coleoptera 19 spp., Hemiptera 16 spp., Ephemeroptera 9 spp., Trichoptera 7 spp., and Collembola 1 sp.), Crustacea 2 spp., Mollusca 19 spp. (Gastropoda 12 spp. and Bivalvia 7 spp.), and Annelids 11 spp. (Oligocaeta 1 sp. and Hirudinea 10 spp.). Sajipo (St.G) and Jjokjibeol (St.H) areas yielded relatively larger numbers of species, 54 spp. and 53 spp., respectively, while more than 40 species occurred at most other sites. Based on quantitative sampling (0.5m${\times}$2m), aquatic insects (88.0％), particularly chironomids in Diptera (61.0％), occupied major proportion of the total individuals of benthic macroinvertebrates, while Mollusca (5.3％), Annelida (3.5％), and Crustacea (3.2％) occupied minor proportions. In standing water areas, diverse groups of benthic macroinvertebrates such as chironomids, demselflies, aquatic bugs, aquatic beetles, crustaceans, and gastropods were dominant in terms of individual number; in the running water areas, on the other hand, chironomids and baetid mayflies were dominant. However, gastropods, i.e. viviparids, were the dominant group of benthic macroinvertebrates in most study areas in terms of biomass. Dominance indices were 0.22-0.51 (mean$\pm$sd 0.42$\pm$0.09) in autumn, 0.31-0.96 (0.02$\pm$0.23) in winter, and 0.30-0.89 (0.57$\pm$0.18) in summer; diversity indices were 3.50-4.26 (3.80$\pm$0.24) in autumn,1.55-4.50 (3.10$\pm$1.01) in winter, and 1.35-3.77 (2.55$\pm$0.09) in summer. Highly movable or true aquatic benthic macroinvertebyates such as aquatic bugs, aquatic beetles, and gastropods recovered earlier after flood. In the study sites of Upo wetlands, Upo and Sajipo areas showed relatively higher values of average diversity index which may indicate a good habitat condition for benthic macroinvertebrates.