Difference in growth pattern is one of the proximate causes of geographic variation in body size. Such a difference in growth pattern could be explained by two extremes such as genetically determined outcome and phenotypic plasticity or by a combination of these. The Japanese four-lined snake (Elaphe quadrivirgata) on Yakushima Island, Japan, has small body size compared with snakes on the main islands. To examine whether the dwarfism of Yakushima snakes is a consequence of genetic modification or a direct phenotypic response to immediate environmental conditions, E. quadrivirgata from Yakushima and a main island (Shiga Prefecture in Honshu Island), were reared from hatching in a laboratory environment. Data of field surveys were used to examine feeding habits and growth in the wild. Free-ranging snakes on Yakushima had significantly smaller body size and lower growth rate than those in Shiga. Yakushima snakes consumed almost exclusively lizards, whereas frogs were the main prey for Shiga snakes. The proportion of snakes that contained food in their stomach was lower in Yakushima than in Shiga. Available information suggests that the dwarfism of Yakushima snakes cannot be explained by lower survivorship. At hatching, Yakushima snakes of both sexes were larger than Shiga snakes. However, Yakushima and Shiga snakes reached similar body size after approximately 1 year. Captive-reared snakes from the insular dwarf population can reach comparable size to those from the main island, indicating that the growth rate, and hence body size, of E. quadrivirgata is obviously highly plastic. Based on these results, I consider that the dwarfism of Yakushima snakes is attributable to a direct phenotypic response, presumably to food limitation, rather than to microevolutionary changes.