In a lot of squid caught in the same season in one fishing are, the mantle muscles of smaller ones contain more myofibrillar and total protein than the mantle muscles of larger squid. Cooking loss and hardness of the cooked samples are smaller in smaller squid than in large ones. The hardness of the cooked mantle muscles is positively correlated with the length of the mantle and negatively with the contents of total and soluble myofibrillar proteins. No correlation has been found between the hardness of the cooked mantle meat and its collagen contents, solubility, and thermal properties of muscle collagen.