Fish act aggressively towards their mirror image suggesting that they consider it another individual, whereas in some mammals behavioural response to mirrors may be an evidence of self-recognition. Since fish cannot self-recognize, we asked whether they could distinguish between fighting a mirror image and fighting a real fish. We compared molecular, physiological and behavioural responses in each condition and found large differences in brain gene expression levels. Although neither levels of aggressive behaviour nor circulating androgens differed between these conditions, males fighting a mirror image had higher immediate early gene (IEG) expression in brain areas homologous to the amygdala and hippocampus than controls. Since amygdalar responses are associated with fear and fear conditioning in other species, higher levels of brain activation when fighting a mirror suggest fish experience fear in response to fights with a mirror image. Clearly, the fish recognize something unusual about the mirror image and the differential brain response may reflect a cognitive distinction.