Sexual dimorphism was examined in a population of the colubrid snake, Dinodon semicarinatum, from the northern part of Okinawajima Island. This snake exhibited a prominent male-larger sexual size dimorphism (SSD), and this was considered to have resulted from male-male combat for mating opportunity. The size of the head relative to snout-vent length was significantly larger in females than in males, suggesting that segregation for differential food items between sexes, often considered the major cause of SSD in snakes, does not account for the distinct SSD in D. semicarinatum. Patterns and causes of sexual dimorphism in other characters are also presented and discussed.