Of a total of 32 sedentary brown hares (Lepus europaeus) approached across open ground by foxes (Vulpes vulpes), 31 reacted when the fox was 50 m or less from them by adopting a bipedal stance directly facing the fox. Of five sedentary hares approached by foxes from nearby cover, none stood, three moved away and two adopted the squatting (primed for movement) posture. Hares stood before foxes in all heights of vegetation and on 42% of occasions were solitary. Hares did not stand before approaching dogs (Canis familiaris). The functions of this behaviour are considered and competing hypotheses of Predator Surveillance and Pursuit Deterrence are examined by testing predictions against results obtained. The results suggest that by standing erect brown hares signal to approaching foxes that they have been detected.