The acute toxicity of an organophosphorous pesticide, fenamiphos and its metabolites, fenamiphos sulfoxide (FSO), fenamiphos sulfone (FSO(2)), fenamiphos phenol (FP), fenamiphos sulfoxide phenol (FSOP) and fenamiphos sulfone phenol (FSO(2)P), to the aquatic alga Pseudokirchneriella subcapitata and the terrestrial alga Chlorococcum sp. was studied. The toxicity followed the order: fenamiphos phenol>fenamiphos sulfone phenol>fenamiphos sulfoxide phenol>fenamiphos. The oxidation products of fenamiphos, FSO and FSO(2) were not toxic to the algal species up to 100 mg L(-1). Both algae were able to transform fenamiphos, FSO and FSO(2), while the phenols were found to be stable in the incubation media. Bioaccumulation of both fenamiphos and its metabolites was observed in the case of Chlorococcum sp. while only metabolites were accumulated in P. subcapitata. This study demonstrates that (i) the hydrolysis products of fenamiphos, FSOP and FSO(2)P are more toxic to both fresh water and soil algae than their parent chemicals, (ii) further fenamiphos can be transformed and bioconcentrated by these algae. Therefore, contamination of natural environments such as waterbodies with fenamiphos or its metabolites can have adverse impacts on the food chain and associated biota (especially to the primary consumers such as Daphnia) since algae are the primary producers located at the base of the food chain. Further, the finding that the fenamiphos phenols are more toxic to algae highlights the need to consider the transformation products in ecological risk assessment of fenamiphos.