The European catfish, Silurus glanis Linnaeus, 1758, has largely colonized lakes and rivers in Western and Southern Europe over the past 40 years. Recent studies have highlighted a high inter-individual variability in the species with strong adaptations in trophic ecology or habitat use. We report here the first and unexpected occurrence of one European catfish individual in underground waters. One young (estimated body size of 80 cm-1 m) catfish was observed 25 m deep in the Uglas cave (river Herault, Southeast France) at 150 m from the cave entrance. The exploratory behaviour of this individual in a very unusual environment illustrates once again the high adaptive abilities of the species.