Gizzard myosin consists of two m ajor subunits, ‘heavy chains’, with chain masses of about 200000 and two minor com ponents ‘light chains’ with chain masses of 20000 and 17000. Quantitative densitometry indicates that the two types of light chains occur in non-stoichiom etric amounts in a myosin molecule, both in purified myosin preparations and in the whole muscle. Chemical studies on the isolated light chains reveals little chemical similarity between the two types, and preliminary sequence work on the thiol peptides of these light chains further suggests that they may be chemically distinct from those of skeletal and cardiac myosins. Although the exact function of the light chains remains speculative, there is evidence to suggest that the presence of one of the light chains is not required for the hydrolytic activity of this myosin.