This study investigated the removal of methyl parathion by cyanobacteria Microcystis novacekii in culture conditions and evaluated toxicity in terms of EC(50) of the pesticide for the species. This is the first report on the removal and toxicity of methyl parathion for the species that is abundant and easily accessible in Brazilian lakes. The results have shown that Microcystis novacekii is capable of removing methyl parathion, an organophosphorus pesticide, from the culture medium with an extraction rate higher than 90%. Spontaneous degradation was not significant, which indicates a high efficiency level of biological removal. No metabolites of methyl parathion were detected in the culture medium at the concentration levels evaluated (0.10 to 2.00 mg/dm(3)). The mechanisms proposed to explain pesticide removal are bioaccumulation and mineralization. EC(50) for 72 h was 17.60 mg/dm(3), which is much higher than the usual concentrations in surface waters. This indicates that M. novacekii is highly tolerant of this pesticide and may represent an important bioremediation agent of contaminated environments.