Due to the presence of an asymmetrically substituted C atom, dimethenamid [2-chloro-N-(2,4-dimethyl-3-thienyl)-N-(2-methoxy-1-methylethyl)acetamide], a recently introduced N-thienyl chloroacetamide herbicide, exists as two stereoisomers (S and R) having di †ering herbicidal activities as demonstrated with a selection of weeds and L emna minor. The activity of the two isomers was investigated in greater detail with the green alga Scenedesmus acutus and compared to that of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide]. As with alachlor, the S isomer (5 kM) strongly inhibited algal growth and fatty acid desaturation while the R isomer had no e †ect. In short-term experiments (up to 5É5 h), the S isomer and alachlor (100 kM) inhibited [14C]acetate uptake and its incorporation into fatty acids in the same manner, while the R isomer did not. Incorporation of [14C]acetate into a non-lipid fraction of the algae was strongly inhibited by alachlor and the S isomer (100 kM) and only slightly by the R isomer. A 50% inhibition of incorporation of [14C]oleic acid into the same non-lipid fraction was attained with less than 10~7 M of the S isomer while 10~5 M of the R form of dimethenamid achieved only a 40% inhibition. The same stereospeciÐcity of the compound on growth, fatty acid desaturation, acetate uptake and oleic acid incorporation provides strong evidence that dimethenamid may act upon a primary, speciÐc target in lipid metabolism. Furthermore, the comparable biological activities of dimethenamid and alachlor indicate that this target is common to both N-phenyl and N-thienyl chloroacetamide herbicides.