Aquatic ecosystems are major sinks for pollutants which can have adverse effects on biodiversity. Thus, it is important to understand the nature of pollution-induced change in aquatic ecosystems. We show that brown bullheads (Ameiurus nebulosus) may have evolved in response to chronic pollution exposure. We collected adults from the Detroit River (polluted site) and Belle River (control site). Both adults and common-garden raised juveniles were tested for aggression, locomotion, and escape response using consecutive unchallenged (clean) and challenged (polluted) trials. Detroit River fish were more aggressive than Belle River fish when challenged. Furthermore, Belle River fish showed increased locomotion when exposed to pollutants, whereas Detroit River fish were unaffected. The consistent difference in adult and juvenile behaviour across trials suggests a genetic response to pollution. Escape response on the other hand, showed inter-population differences, but no consistency between adults and juveniles, indicating that this behaviour is influenced by non-genetic factors. We discuss our data with respect to the potential adaptation of populations to pollution and the implications for prioritizing remediation efforts.