The material of Spirogyra azygospora Singh was exposed to different doses of UV light ranging from 5.22×10³ ergs/mm² to 52.2×10³ ergs/mm². 52.2×10³ ergs/mm² and 26.1×10³ ergs/mm² proved to be completely lethal and sublethal doses respectively to this alga, coupled with 24 h dark incubation. Sensitivity of the alga to UV light, and effects of different periods of dark incubation (0-24 h) after UV treatment were also studied; such studies being first of their kind on a filamentous green alga. Extent of survival was a dose dependent process, irrespective of the period of dark incubation. UV effects were clearly observed with all the doses administered, even when the irradiated materials were immediately exposed to light without any dark incubation. At the same time, phenomenon of photoreactivation also plays an important role in reversing the adverse effects caused by UV light. A minimum of 12 h dark incubation was found to be necessary for non-functioning of the phenomenon. Relative sensitivity of three desmid taxa, Closterium lanceolatum f. minor f. nov., Cosmarium cyclicum and Euastrum cornubiense f. major f. nov., to UV light was assessed on the basis of their percentage survival to different doses of UV light. The doses of 78.3×10³ ergs/mm³ and 156.6×10³ ergs/mm² proved completely lethal to Closterium lanceolatum f. minor f. nov. and Cosmarium cyclicum respectively, whereas the same exposure (156.6×10³ ergs/mm²) proved to be sublethal dose for Euastrum cornubiense f. major f. nov. The observations in the present study are discussed in the light of earlier existing data on other groups of algae.