We present results of an experiment performed in Lake Baikal at a depth of ～ 1 km. The photomulti-pliers of an underwater neutrino telescope under construction at this site were illuminated by a distant laser. The experiment not only provided a useful cross-check of the time calibration of the detecter but also allowed us to determine inherent optical parameters of the water in a way that was complementary to standard methods. In 1997 we measured an absorption length of 22 m and an asymptotic attenuation length of 18 m. The effective scattering length was measured as 480 m. By use of ＜cos θ＞ = 0.95 (0.90) for the average scattering angle, this length corresponds to a geometric scattering length of 24 (48) m.
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