*Department of Botany, Career College Bhopal India

**Department of Botany, Gandi P.College Bhopal MP India

 Environment is always kept on changing over a period due to the changing climatic and physiographic factors. These factors influence marked effects upon the genetic and phenotypic machinery that brought about by various intracellular and extracellular factors which operating throughout the life history of organisms. These factors elicit genetic responses that ultimately manifest themselves through a morphological and physiological modification of the phenotypes. The genus Scenedesmus is well known for its extreme phenotypical variability and plasticity. This study has revealed the phenotypic response of Scenedesmus ovalternus to UV exposures. The 15 min. of UV exposure cause decrease in cell width and cell length from their definite cell dimension. As the time of exposure increases the cell dimension decreases, after 90 min. of continuous UVR treatment the Scenedesmus cell length 12.2 µm and width 5.5µm were measured which extensively reduced from 16.5µm×9.5µm. These findings clearly signify the difference in Scenedesmus ovalternus morphology after the long term exposure of UVR and the mean cell length 14.7µm, cell width 7.7µm were measured with observation of S.ovalternus colonies and cells shrinkage. The number of cells per colonies was fixed during the course of experiment but the chlorophyll `a' content and cell dimension was greatly reduced.