1. Myosins from an adult individual and embryos of Salmo trutta L. in different stages of development were isolated. Their light chain composition was investigated by polyacrylamide gel electrophoresis using two systems: tris-glycine buffer, pH 8.6, containing 8 M urea and 100 mM sodium phosphate buffer, pH 7.0, containing 0.1% SDS. 2. Both types of myosin are composed of three light chains with mol. wt 26200 D, 18300 D, 16800 D. 3. Almost no changes in electrophoretic patterns were discovered between the separate stages of development, except for the intensity of the light chain lc3, which increased gradually during miogenesis.