The effect of ultraviolet radiation into the Lake Chapala trophic processes was investigated in this study. Responses of bacterial populations to changes in UV radiation exposed water were significantly greater (bacterial biomass increased 57% and cell concentration increased 92%) compared to those populations that were grown in water covered by glass as UV blocker. Measurements for penetration of ultraviolet radiation in the water column of Lake Chapala were made at midday and to a depth of 0.45 m in one of the clearest parts ofthe lake (Station 11). Ten per cent of the UV radiation that reached the surface ofthe lake was still present at 0.2 m of depth. The extinction coefficient was 10.1 m. For a lake with low phytoplankton productivity, the supply of organic carbon via photolysis ofrefractory material may be an important supplement to bacteria in the water column.