Surveys on the natural history of two diurnal colubrid snakes, Elaphe quadrivirgata and Rhabdophis tigrinus, were made on Yakushima Island, Japan. Two color morphs, normal striped and melanistic, were observed in E. quadrivirgata, and the frequency of the latter morph was significantly greater in 1998 (83.3%) than in 1981 (56.4%). Regurgitated stomach contents of E. quadrivirgata consisted solely of reptilian prey, and the skink Eumeces latiscutatus was by far the most frequent prey item. This result makes a sharp contrast to those of previous studies dealing with populations of this species in central and northern Japan, where the snake preys mainly on anurans. On the other hand, examination of stomachs of R. tigrinus yielded only anuran prey. In Elaphe quadrivirgata, both snout-vent length and body mass in the Yakushima population were smaller than those in a population from central Japan, whereas the size variables for the Yakushima population appeared greater than those in the central Japanese population in R. tigrinus. Body temperature was significantly higher in E. quadrivirgata than in R. tigrinus.