Despite extensive research, the function and adaptive significance of melanism remain controversial.In snakes, melanistic individuals enjoy a thermal advantage compared with normal-colored individuals due to superior thermoregulatory capabilities. The hypothetical consequences of this thermal advantage are that melanistic individuals have longer daily and seasonal active periods, and thus collect more food, resulting in a higher growth rate and larger body size. To test the generality of this hypothesis, I made intermorph comparisons of body size using a melanistic/striped color-dimorphic snake (Elaphe quadrivirgata) on Yakushima Island, Japan. Melanistic individuals were not significantly larger in body size than striped individuals in either males or females, and the largest individual was a striped morph in both sexes. Thus, the pattern predicted based on the hypothetical consequences of the thermal advantage of melanism was not observed in E. quadrivirgata on Yakushima Island. Based on this coupled with the results of a previous survey on the thermal ecology of the snake, I conclude that melanistic individuals of E. quadrivirgata on Yakushima Island may benefit from fast body warming, which shortens the time of basking, but this benefit is not sufficient to realize larger body size compared to that of striped individuals, and/or such a benefit is not realized in the hypothesized way. The thermal advantage of melanism in nature may be more limited in scope than has been assumed.