Introduced populations often exhibit rapid phenotypic changes following colonization of new environments. These changes, which often contribute to the successful establishment and spread of introduced species, may result from evolution or phenotypic plasticity. We studied variation in adult body size across elevational gradients in native (Puerto Rico) and introduced (Hawaii) populations of Coquis (Eleutherodactylus coqui). To explore the possible mechanisms underlying variation in adult body size, we conducted a laboratory common-environment experiment to study the effects of temperature (19°C and 25°C), elevation (<300 m and >700 m), and area (two in Puerto Rico and one in Hawaii) on five size-related life history traits: clutch size, egg size, hatching size, size at 30 days, and growth rate. In the field in both Puerto Rico and Hawaii, body size was positively correlated with elevation, which is negatively correlated with temperature, but the magnitude of the slope was greater in Puerto Rico than in Hawaii. In the laboratory, egg size, hatching size, and body size at 30 days were positively correlated with elevation for populations from Puerto Rico and Hawaii. Egg size, hatching size, and body size at 30 days were negatively correlated with temperature for all populations. Clutch size and growth rate were positively correlated with elevation for populations from Puerto Rico but not for populations from Hawaii. Furthermore, both low and high elevation populations from Hawaii had life history traits more similar to low elevation populations than high elevation populations from Puerto Rico. Temperature effects in the laboratory suggest that plasticity in response to temperature contributes to the variation in adult body size with elevation in both Puerto Rico and Hawaii. However, temperature-induced plasticity cannot explain the difference in slopes between Puerto Rico and Hawaii, because temperature varies across elevations to the same degree in both regions. Variation in growth rate paralleled that for adult body size and, if heritable, provides a possible mechanism for the observed differences in adult body size across elevations between Puerto Rico and Hawaii.