Lake Winnipeg water is usually turbid, and little light penetrates into the lake. Secchi disc visibility varied from 0.1 t 1.0 m in the south basin and 0.5 to 2.6 m in the north basin. Horizontal extinction coefficients calculated from transmissometr (beam transmittance) data were high (12-46/m) in the south basin and lower (0.5-3/m) in the central part of the north basin. Vertical extinction coefficients were calculated from percentage of surface light reaching the sensor of the submersible photometer ell, and these values were in the range 1-5 m in the south basin and narrows, and 0.5-1/m in the center of the north basin.