Electron spin resonance (e.s.r.) observations of squid melanin have been conducted over the temperature range 500 degrees K to 4.2 degrees K, and the effect of various chemical treatments of the melanin upon the e.s.r. spectrum has been studied. The findings have shown that the paramagnetism of this melanin follows the Curie Law from 500 degrees K to 4.2 degrees K, that the spin signal can be eliminated by the addition of Cu(++) to the melanin, and that the optical and e.s.r. absorptions of melanin are independent since either can be reduced or eliminated without affecting the other. Similar studies on synthetic melanins produced by autoxidation or by enzymatic oxidation of a number of biphenols were carried out. It was found that the e.s.r. signals of these synthetic melanins were strikingly similar (with respect to line width, line shape, and g-value) with those of squid melanin. It is concluded that the unpaired electrons observed are associated with trapped free radicals in the melanin polymer, that the biosynthesis of melanin may involve a free radical mechanism, and that these physical data are in accord with the concept of Nicolaus that melanin is a highly irregular, three-dimensional, polymer.