Nowadays, the bio-monitoring is an important part of the water quality monitoring. In particular, the aquatic organisms of benthic macro-invertebrates, zooplankton and phytoplankton are commonly used as bio-indicators in the water quality monitoring and environmental impact assessment for rivers. ponds and lakes. Among them, the benthic macro-invertebrates are more popularly used because they have a high liversity of species, high density of individuals, high adaptability to the change of environment, a widespread distribution, some species with long-life, and slow moving. They are also easy of sampling and taxonomic identification. In 2011, samples of benthic macro-invertebrates were collected at 11 sites during bIr field surveys in Vam Co Dong river, Long An province. the authors recorded a total of 41 species in six classes and three phyla. Bivalve molluscs and aquatic insects are the most diverse groups in terms of species composition. Whereas, worms, river mussels, clams and mosquyto larvae are the species groups wilh a high density of individuals at the sampling sites. Bio-quality of the bottom water of Vam Co Dong river is polysaprobe to oligosaprobe.