The carotenoid composition of crayfish exoskeleton before and after heat-treatments was determined in order to explain the mechanism of thermal denaturation of carotenoprotein. (1) The reddening of exoskeleton by boiling resulted not only from the splitting of carotenoid-protein link, but also from some structural modifications of the protein. (2) There was no evidence of the yield of astacin caused by heating of exoskeleton. (3) Violent heating decomposed astacin more heavily than other carotenoids in the exoskeleton.