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ABSTRACT1. Invasions are a main concern for conservationists. Identification of causes that raise or promote an invader's success is one of the first steps for preventing and/or fighting against invasions.2. In the Brazilian River Paraiba do Sul, juveniles of invasive tilapia Oreochromis niloticus were detected associated with degraded habitats (mainly pollution from various sources). The relative abundance of juveniles of the competitor native species pearl cichlid Geophagus brasiliensis, morphologically and ecologically very similar, was negatively associated with habitat degradation.3. A species-specific molecular marker was employed to permit unambiguous identification of these small juveniles (2-3 cm long), which represent the most vulnerable life stage of both species.4. It is suggested that adaptation to degraded habitats is one of the characteristics of invasive species, and consequently habitat conservation and restoration may help to prevent fish invasions.