The effect of solar ultraviolet radiation (UVR) on viability of natural bactenoplankton assemblages from Antarctic waters, as well as on 2 cultures of isolated bacterial strains (Acinetobacter sp. and BaciUus sp.), was determined by both in situ and temperature-controlled incubator experiments. When natural assemblages were incubated in situ at 0.5 m depth, the mean percentage survival fractions (of the bacteria forming colonies on agar) were 13% when the sample was exposed to all UVR, 27 % when UV-B radiation was eliminated with a prefilter, and 85 % when all UVR was excluded. The magnitude of UVR-induced inhibition decreased with depth so that there was no significant inhibition at 9.5 m. There was very little effect of photosynthetically available radiation (PAR), even at 0.5 m depth. The loss of viability due to UVR or PAR was much greater for the 2 isolated strains than for the natural bacterial assemblages. Exposure of the Bacillus sp. to incident PAR, PAR + UV-A, and PAR + UV-A + UV-B resulted in survival values of 9, 0.4, and 0.1 %, respectively; when irradiance to the sample was reduced to about 3% of the ~ncident value, the corresponding values were 80. 50, and 24 %, respectively. With Acinetobactersp., the corresponding values were 60, 13, and 1.5 % with direct exposure to solar radiation, while at 1 2 % of incident radiation no inhibition by either UVR or PAR could be detected . The SOS-repair system could be induced in both bacterial strains studied with the result that the loss in viability due to UVR radiation was much reduced, but both strains still showed some loss of viability when compared to the control samples.