The structural stabilities of all the familiar proteolytic fragments of myosin have been investigated in melting studies over the pH ranges 5.5-7.0 in 0.5 M KCl. All fragments except subfragment 2 undergo a melting transition manifested by the cooperative uptake of protons in the temperature range 34-47 degrees C, and these fragments experience an increase in transition temperature, Tm as the pH is increased. Subfragment 2 undergoes a melting transition in the 43-55 degrees C range, manifested by the dissociation of protons, and it experiences a decrease in Tm as the pH is increased. These results suggest that pH changes can modulate the relative stabilities of the light meromysin, subfragment-1, and subfragment-2 regions of the myosin molecule.