Territorial animals sometimes use conspicuous natural landmarks as boundaries to their territories. The utilization of territory-demarcating landmarks may have evolved to limit the costs of territorial defence, since the adoption of clearly defined boundaries by opponents in adjacent territories can reduce the overall rate of aggressive encounters, which can be energetically expensive or might result in injury. Here the role of artificial landmarks as boundaries was tested in territorial male rose bitterling (Rhodeus ocellatus), a fish with a resourcebased mating system. Pairs of size-matched territorial males were permitted to interact for short periods in an otherwise featureless aquarium with an obvious landmark at the shared boundary of their territory either present or absent. The presence of the territory-demarcating landmark significantly reduced both the frequency of territorial incursions by males into adjacent territories and the rate of territorial displays. Males showed individual differences in their propensity to enter the territory of a rival, irrespective of the presence of a territorydemarcating landmark. These results suggest that the cost of defence of a territory may be reduced by utilizing territory-demarcating landmarks, in accordance with the predictions of theoretical models.