The community of crustacean zooplankton in lakes can show temporal vertical variations in composition and abundance due to the influence of exposure to natural ultraviolet radiation. The aim of the present study was to analyse the zooplankton composition both at the surface and in the water column, and at different times during the day and night, in a northern Patagonian mountain lake (Lake Tinquilco, 39°S, 763 m a.s.l.) in Chile. Eight samples were collected in two days: four samples between the surface and 5 m depth, and four between the surface and 20 m depth. Based on these data, the corresponding values of the Shannon diversity index were estimated. In samples collected at 0-5 m depth, low species numbers were found in comparison with samples collected at 0-20 m. At 0-5 m, the highest biodiversity was observed in the evening, whereas at 0-20 m depth the highest biodiversity was observed in the morning and early afternoon. As a possible cause for this variation, the exposure to natural ultraviolet radiation would be the most probable, since that harmful radiation can penetrate into the superficial part of the water column.