Abstract: The present study described the sensitivity of two strains of Scenedesmus obliquus of different origin to allelopathic activity of the aquatic macrophyte Stratiotes aloides. The two algal strains were grown in batch cultures with different concentrations of aqueous acetone extract of S. aloides for 7 days. Growth and antioxidative biomarkers of Scendesmus strains were measured at the end of incubation period. The results showed that the two strains of S. obliquus responded differently to the allelopathic activity of the macrophyte. The extract of macrophyte inhibited the growth and increased the lipid perooxidation and activities of antioxidative enzymes of Scenedesmus strain (SZA1) originating from Stratiotes-not-containing pond. In contrast, strain SZA2 originating from Startiotes containing pond showed no significant change in the growth or physiological parameters upon exposure to Stratiotes extract. This study suggests that Scendesmus strains originating from the same habitat of Startiotes are less sensitive to its allelopathic activity than strains that had not been in contact with Startiotes and these strains might have developed resistance against allelopathic substances of the macrophytes by co-evolution.