We tested for environmental factors that may lead to balancing selection and to the maintenance of a genetic polymorphism at the enzyme locus lactate dehydrogenase B (LDH-B) in the pool frog, Rana lessonae. We raised tadpoles individually in a factorial experiment in which we manipulated temperature, food level, and food quality. The only statistically significant difference among LDH-B genotypes was in growth rate, with the heterozygote performing best. Although the difference was not significant, heterozygotes also tended to perform best for size at metamorphosis. However, heterozygotes did not perform best in terms of other traits (age at metamorphosis and rates of survival and metamorphosis), where differences among LDH-B genotypes were also not significant. The size of the effect of LDH-B genotype depended on the environment, which suggests that the locus may be selectively neutral in some environments. There were no genotype-environment interactions in the sense that reaction norms along environmental gradients did not cross. When we raised tadpoles in groups, e/e homozygotes had a significantly higher body mass and developed at the significantly highest rate. In addition, there may be a trade-off between larval and adult performance: adult frogs show a different ranking in performance of LDH-B genotypes than tadpoles do. These results suggest that this genetic polymorphism is maintained through heterozygote advantage, possibly in conjunction with antagonistic pleiotropy.