Abstract: In the meromictic alpine Lake Cadagno a dense layer of phototrophic bacteria, mainly Chromatium okenii and Amoebobacter purpureus, develop annually at the chemocline at about 10 to 11 m depth. Radiometric spectral profiles of the incident sunlight demonstrate different attenuation coefficients in the mixolimnion and in the chemocline not only for the visible light effective at each depth (photosynthetically available radiation), but also for selected photosynthetically active wavelengths used by oxygenic and anoxygenic phototrophs. Phototrophic bacteria sampled from the upper part of the layer at the maximum of cell concentration were incubated in transparent bottles at the sampling depth and at a lower depth where the light intensity is only a few percent of the one at the sampling depth. Within 4 h the specific bacteriochlorophyll concentration (Bchl protein−1) increased up to 50% depending on the difference in light intensity between the sampling and the incubation depth. The specific bacteriochlorophyll concentration in the upper part of the layer remained constant (53.0 mg Bchl g−1 protein, S.D. = 4.8) in spite of large changes in cell concentrations in the lake water over the season. These observations illustrate the phenomenon of light-regulated pigment synthesis under natural conditions.