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In European streams, native brown trout (Salmo trutta) feed primarily on aquatic prey but consume a higher proportion of terrestrial prey in sympatry with non-native brook trout (Salvelinus fontinalis). This is a rare example of diet convergence that may be associated with changes in diel activity or aggregation pattern by brown trout in sympatry. We recorded the activity and positions of brown trout from two origins and in two competition modes (allopatry versus sympatry, four combinations) placed in replicated stream enclosures for 29 days to test these hypotheses. Brown trout originating from or placed in sympatry were more diurnal and aggregated than those originating from or placed in allopatry. Changes in the diel activity of brown trout placed in a novel competition mode occurred progressively throughout the study. Thus, brown trout show strong behavioral flexibility in response to the non-native competitor and can revert to allopatric behavior when brook trout is removed from the system. These behavioral adjustments may have unsuspected effects on food webs and ecosystem functioning, which deserve further attention.