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The river has many benefits for the needs of human life, but most of the 80% of the rivers are heavily, lightly and moderately polluted including the Mandar River which is widely used by the surrounding community. Water quality monitoring that can provide information by identifying aquatic biota living in rivers, namely Biotilik. The purpose of this study was to determine the level of water quality pollution in the Mandar River using the biolotic method. This research is descriptive with a survey approach that identifies the Mandar River Biota as an indicator of water pollution in Petoosang Village, Alu District. The data analysis was carried out by starting from the identification of macroinvertebrates obtained, then grouping the biota into 2 groups, namely EPT (Ephemeroptera, Plecoptera, and Tricoptera) and non-EPT, then
calculating the Water Pollution Index. The results showed that the Mandar River in Petoosang Village, Alu District, was in the medium polluted category