Résumé
1. The ecological and economic costs of introduced species can be high. Ecologists try to predict the probability of success and potential risk of the establishment of recently introduced species, given their biological characteristics.2. In 1990 gudgeon, Gobio gobio, were released in a drainage canal of the Rhône delta of southern France. The Asian topmouth gudgeon, Pseudorasbora parva, was found for the first time in the same canal in 1993. Those introductions offered a unique opportunity to compare in situ the fate of two closely related fish in the same habitat.3. Our major aims were to assess whether G. gobio was able to establish in what seemed an unlikely environment, to compare populations trends and life‐history traits of both species and to assess whether we could explain or could have predicted our results, by considering their life‐history strategies.4. Data show that both species have established in the canal and have spread. Catches of P. parva have increased strongly and are now higher than those of G. gobio.5. The two cyprinids have the same breeding season and comparable traits (such as short generation time, small body, high reproductive effort), so both could be classified as opportunists. The observed difference in their success (in terms of population growth and colonization rate) could be explained by the wider ecological and physiological tolerance of P. parva.6. In conclusion, our field study seems to suggest that invasive vigour also results from the ability to tolerate environmental changes through phenotypic plasticity, rather than from particular life‐history features pre‐adapted to invasion. It thus remains difficult to define a good invader simply on the basis of its life‐history features.