The myosin contained in white and red muscles of herring (Clupea harengus harengus) was purified, and its subunit composition analyzed by electrophoretic techniques. The only myosin isoform present in red muscles was made up of one type of heavy chain and two types of light chain. The native myosin from white muscles migrated as one wide band. Analysis of the extracts by SDS/glycerol/PAGE from white muscles revealed one main type of heavy chain. Light chains were identified by SDS-PAGE analysis of electrophoretically purified myosin, and two-dimensional electrophoresis of the extracts demonstrated differences in the light chain composition of white and red muscles. Using this methodology, light chain polymorphism was detected in white muscles among members of the same species.