The ecological relationships between two sympatric vipers (Vipera aspis and Vipera ursinii) were studied at a mountainous area in central Italy (Gran Sasso, Abruzzo), between spring 2004 and autumn 2006. The two species differed significantly in their altitudinal distribution: V. ursinii being confined to the higher elevations (above 1650 m elevation), and V. aspis being present from the lowest to the highest altitudes (1200–1900 m). We recorded a wide sector of approximately 250 m elevation (between 1650 and 1900 m elevation) where the two species overlapped. In these areas, the two species exhibited significant difference in microhabitat use and diet composition. Male and female V. aspis were larger (SVL) than male and female V. ursinii. Furthermore, the two species exhibited a significant difference in sexual size dimorphism, with females being larger than males in V. ursinii, whereas no such difference in SVL was observed in V. aspis. However, we did not observe any difference in SVL of male and female vipers when comparing sympatric and allopatric populations. Overall, the potential for competition between these two species appears to be low because of their clearly different realized ecological niches.