1. Myosin from the thin-filament regulated flexor muscle of lobster contains 2 moles of each of 2 light chains. 2. The Lb 1 light chain of 19,000 daltons which can be removed by DTNB is heavier than the DTNB light chain of chicken. The Lb 2 light chain of 17,000 daltons can be removed with urea. 3. On electrophoresis in 8 M urea (pH 8.7) the Lb 2 light chain migrates with a mobility similar to that of chicken A2, but the Lb 1 migrates significantly faster than any of the chicken light chains. 4. In lobster, the DTNB treatment destroys the Ca and K-EDTA ATPase activity of lobster myosin.