Within the context of a study on tourist disturbance affecting Mediterranean Mouflon {Ovis gmelini musimon x Ovis sp.) in the Caroux-Espinouse massif (Hérault, France), we assessed the sensitivity of this species towards the presence of a dog (Canis familiaris) taken along by two hikers. With the help of standard experimental disturbance events we measured the distance at which Mouflons spotted a source of disturbance, modified their activity and became vigilant, as well as their escape distance. Three sources of disturbance were tested in the springs of 1996 to 1998 on the Caroux plateau, one of the areas in the massif with the highest tourist pressure : approach by two hikers, approach by two hikers with a leashed dog and approach by two hikers with a dog off leash. If the median Mouflon escape distance increases very significantly in the presence of a dog, the fact of keeping him on leash or allowing him to run does not make much difference : from 70 m (N = 95) when the two hikers are alone, the escape distance goes up to 100 m (N = 21) and 98.5 m (N = 36) when these persons are with a leashed or unleashed dog, respectively. Besides, the presence of two people with a dog may modify the behaviour of the Mouflons in an area of about 7.5 ha around them, whereas if the two people are alone under the same conditions, the disturbed area is only of about 3.7 ha. This increased sensitivity of Mouflons to the presence of a dog indicates a great stress, which is a long-term source of perturbation that could affect their population dynamics, like in the case of chases by dogs off leash (93.6 % of cases). Therefore, and in the context of further development of ecotourism in the massif, the control of such disturbances must henceforth be envisaged.