This paper reports the results of an investigation into the size and shape of the low molecular weight subunits (light chains) of myosin from several animal species. Hydrodynamic, analytical gel filtration, and fluorescence anisotropy decay measurements indicated that these light chains could be represented by a general ellipsoidal model having a longest axis of about 100 +/- A. Investigation into the stability of the internal structure of the scallop regulatory light chain was carried out by studying the effect of pH, ionic strength, temperature, and guanidine hydrochloride on its circular dichroic spectrum. The nearly complete insensitivity of the circular dichroic spectrum to pH, ionic strength, and temperature variations from 4 to 70 degrees C indicated that this subunit contained regions of very stable structure which probably exist when it is bound to myosin.