The degradation of an organophosphorus pesticide, fenamiphos, by different species of five green algae and five cyanobacteria was studied. All the species tested were able to transform fenamiphos to its primary oxidation product, fenamiphos sulfoxide (FSO), while the majority of these cultures were able to hydrolyze FSO to fenamiphos sulfoxide phenol (FSOP). Fenamiphos sulfone phenol, FSOP, and FSO were detected in the culture extracts of these algae and cyanobacteria. This is the first report on the biodegradation of a toxic pesticide, fenamiphos, by cyanobacteria. The ability of these algae and cyanobacteria to detoxify fenamiphos can be gainfully used in bioremediation of this pesticide and its toxic metabolites.