In Western Europe, old abandoned mining sites and quarries are often of high biodiversity and conservation value due to the presence of a number of endangered species. In the southern part of Belgium (Wallonia), many ancient quarries near the river Meuse are rather small and were abandoned from 50 to more than 100 years ago. In 2003, we collected 26 ant species by pitfall trapping in four of these quarries. In addition to common ones, several rare species, usually associated with mesomorphic to xeromorphic grasslands, were found in high numbers. Quarries undergoing forest succession were dominated by eurytopic species and by species typical of wet shadowy places, a fauna far less valuable in terms of nature conservation. Therefore, we suggest a management that halts further forest succession of open mesomorphic and xeromorphic habitat patches in these quarries. To assess and monitor the nature value of the ant fauna of these sites, we propose a so-called habitat preference approach, wherein each species is assigned to one of the following three habitat preference categories: (1) eurytopic, (2) bound to wet shadow-rich habitats, or (3) bound to dry open habitats. The stenotopic species of the last category are all endangered in Belgium and of high conservation value. The proportion of the total number of captured specimens included in the latter habitat preference category group is strongly reduced as scrub and tree encroachment advances. This proportion can therefore be used as a proxy to monitor the effects of management measures that prevent further forest succession.