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Gold mine operations release arsenic pollutants in environment. The present study investigated the diversity of bacterial communities in the arsenic-contaminated tailings dam effluent (TDE) of Zarshuran gold mine, Takab, northwest of Iran. The bacterial communities were examined using the next-generation sequencing method (Illumina) targeting the V3-V4 region of 16S rRNA genes. Bacteroidetes (50.3%), Proteobacteria (45.49%), Actinobacteria (1.14%) and Firmicutes (1.08%) constituted dominant phyla in the TDE. Its diversity was analyzed, and compared with that of three previously-studied arsenic-contaminated groundwater (GW) microbiomes. The raw sequencing data were analyzed in QIIME2. The prevalent taxonomic groups observed in all of the samples belonged to Proteobacteria (8.06-45.49%), Bacteroidetes (1.85-50.32%), Firmicutes (1.00-6.2%), Actinobacteria (0.86-5.09%), Planctomycetes (0.05-9.37%) and Cyanobacteria (0.6-2.71%). According to Principal Coordinates Analysis (PCoA), the TDE and GW samples were divided into high and low groups, based on their arsenic content, respectively. The average numbers of observed alpha diversity indices, i.e. Pielou’s evenness and Faith’s phylogenetic diversity, were not significantly different (P=0.18, Kruskal-Wallis test). According to the beta diversity analysis (unweighted), the observed operational taxonomic units (OTUs) and the diversity of the bacterial populations were not significantly different in the TDE, compared to the GW samples (PERMANOVA, P=0.276, 999 Permutations).