The majority of anuran amphibians shows a sexual size dimorphism, with females being larger than males. It is unknown, however, whether this difference already develops during the larval stage or whether it is exclusively the result of postmetamorphic differences in growth. Using tadpoles of the European water frog Rana ridibunda, I compared larval life-history traits between sexes and found no significant differences in larval period and size at metamorphosis, indicating that the sexual size dimorphism is only established after metamorphosis. Rana ridibunda was used in this study because it is one of the parental species of the hybrid frog R. esculenta, for which this finding is particularly relevant. Within the hybridogenetic R. esculenta complex, different mating combinations produce either all-female offspring or offspring with even sex ratios. Comparisons of larval performance between crosses therefore often rely on the assumption that male and female tadpoles do not differ in the traits measured. My result suggests that for larval period and size at metamorphosis, this assumption is justified in European water frogs.