A cyanobacteria species, Anabaena sp. PCC 7120, was tested to assess its biotransformation ability on two widely used insecticides, aldrin and chlorpyrifos-methyl, in the culture medium. The blue-green alga metabolized aldrin mainly to dieldrin by an epoxidation reaction with the participation of cytochrome P450-dependent monooxygenase in the cyanobacteria. The blue-green alga also produced chlorpyrifosmethyl oxon as a primary metabolite from chlorpyrifos-methyl via a desulfuration reaction, presumably conducted by cytochrome P450-dependent monooxygenase. Therefore, two insecticides might be possibly dissipated by cytochrome P450-dependent monooxygenases in the blue-green algae in the contaminated environments.