One of the most important factors that affect the operation efficiency of sequencing batch reactor (SBR) technology is bacterial viability and biomass activity. The acute toxicity of three heavy metals to four dominant strains of sequencing batch reactor (Pseudomonas, Aeromonas, Enterobacter, and Bacillus) was investigated using a resazurin bioassay. After exposing the bacterial strains to soluble compound of Hg, Cd, and Pb, at more than five selected concentrations, the median effective concentration (EC50) and the mortality rate values were calculated. Large differences were observed in sensitivities of the four bacterial strains to the metals. Pseudomonas showed the highest sensitivity for Cd (EC₅₀ = 0.06 μmol/L) and Hg (EC₅₀ = 11.75 μmol/L), while Aeromonas showed the highest sensitivity for Pb (EC₅₀ = 48.27 μmol/L). Considering the EC50 test results, it was concluded that Pseudomonas and Aeromonas are excellent and reliable bioindicators for assessing the toxicity of water and wastewaters polluted by Cd, Hg, and Pb. The rapidity (30 min) and simplicity of the resazurin bioassay procedure enable this enzymatic test to be used in toxicity assessment of small and decentralized wastewater treatment plants (WWTPs).