Inactivation of SP_1 virus(solanacearum phage α) suspended in distilled water is produced by light of shorter wave lengths than about 3100Å. The inactivation curve approaches a straight line in log graph for high doses while it shows some deviation at low doses. The fact that the rate of inactivation of the virus exposed to ultraviolet light is a logarithmic function of the dose of irradiation, indicates that one-hit mechanism of inactivation, one quantum being the effective inactivating hit. Ultraviolet inactivated virus is adsorbed and kills the host bacterium, Bact. Solanacearum Smith S strain.