AbstractMicroorganisms have demonstrated several attributes that make them attractive for use in quick screening of effluents and chemicals for toxicity. The aim of this study was to evaluate the acute toxicity and mutagenicity of industrial effluents from São Paulo State using short‐term microbial bioassays. Samples of industrial effluents and receiving waters were analyzed for acute toxicity by the Microtox system, a motility test usingSpirillum volutans, growth inhibition ofPseudomonas fluorescens, and dehydrogenase assay; for mutagenicity, these samples were analyzed bySalmonella typhimurium(Ames test),Escherichia coliWP2, andSaccharomyces cerevisiaereversion mutation assays. Among the acute toxicity assays carried out in this study, the Microtox andS. volutanstests showed good sensitivity and general good agreement with theDaphnia similisassay, which demonstrates that these tests are potentially useful as toxicity indicators for the industrial effluents and receiving waters considered. In relation to mutagenicity assays, good results were obtained with the three methods tested. The detection of mutagens in the industrial effluents considered indicates that some constituents of these waste waters discharged in receiving waters can cause adverse biological effects and could be deleterious from a public health standpoint. The data of this research emphasize the importance of acute toxicity and mutagenicity assays as supplementary approaches for a rapid and efficient action in water pollution control, and for evaluation of potential toxic chemical effec