The introduction of a species to areas outside its native range can result in ecological and genetic changes of evolutionary significance. The frog Eleutherodactylus coqui was introduced to Hawaii, from Puerto Rico, in the late 1980s and has lost genetic variation in mitochondrial DNA. The extent to which founder effects have influenced phenotypic variation in the introduced range is unknown. In this study I compared phenotypic variation in life-history traits, advertisement calls, and stripe patterns among introduced and native populations of the frog Eleutherodactylus coqui. I also conducted laboratory experiments to determine the influence of genetics and temperature on trait variation. Body size in wild populations was positively correlated with elevation in both ranges, but the slope of elevation on body size was greater in Puerto Rico than in Hawaii. Advertisement call frequencies and rates were negatively correlated with elevation but duration was positively correlated with elevation. Frequencies were correlated with body size, but rate, duration, and intensity were not. Color patterns are more variable in Puerto Rico than Hawaii and appear to be maintained by balancing selection in Puerto Rico. Lab results indicate that body size is negatively correlated with temperature, which may explain Bergmann's rule in the field, but patterns of intrinsic growth rate may explain differences in the effect of elevation between Hawaii and Puerto Rico. Body size appears to explain most of the variation in call frequencies, whereas temperature explained most of the variation in rate and duration. Color patterns appear to be determined by a single locus with five alleles. Founder effects appear to explain the difference between Hawaii and Puerto Rico in color pattern variation and in clinal variation in body size and call frequencies. The loss of genetic variation in these traits is likely to have evolutionary consequences for this species in Hawaii.