Pesticides have been identified as one of the major reasons for the worldwide decline of amphibian populations. In terms of aquatic amphibian exposure, most ecotoxicological studies concentrate on testing tadpoles in Gosner stage (GS) 25. To examine the representativeness of GS25, we exposed five aquatic stages of European common frogs (Rana temporaria) to the viticultural fungicide folpet (Folpan^® 80 WDG) in a dose–response setup. The 96-h mortality data identified the hatchling stage GS20 as the most sensitive aquatic stage (LC₅₀ of GS15: 2.68 mg/L, GS20: 1.01 mg/L, GS25: 1.22 mg/L, GS36: 2.11 mg/L, GS42: > 2.6 mg/L). Because these results suggest that the commonly used stage is not sufficient for a protective environmental risk assessment of pesticides, comprehensive investigations are needed to provide adequate regulatory guidance.