Abstract. P. Devillers, R. C. Beudels, J. Devillers‑Terschuren, P. Lebrun, J.-P. Ledant & E. Srusiaux.- An environmental monitoring scheme based on bio‑indicators [Natural. belges (Orchid. 5) 71 (1990): 75-98]. The semi‑natural, often unstable and insularized character of most of the biologically important areas of Europe makes management and the corollary need for monitoring a universal requirement of nature conservation and environmental preservation. An environmental monitoring system based on the population trends of a few groups of organisms with high bio‑indicator value was established in 1989 by the Ministry of Environment of the Wallonie Region in southern Belgium. Birds, dragonflies, amphibians and reptiles, butterflies, carabid beetles, orchids and lichens were chosen as indicator groups, to which was added a network of sites on which the populations of all plant species is to be monitored. Regional or local indices are derived from the annual variations of weighted richness in those groups of a number of sites selected to form a coherent sampling network. The weighting coefficient chosen for each species recorded is a vulnerability coefficient which takes into account the spread and configuration of its geographical distribution, the spatial frequency of occurence of its preferred habitat, its abundance within that habitat and its long-term population trends at the level of resolution of either the region or the European Community. The present paper describes the detailed methodology, sampling network and operational parameters for one of the bio‑indicator groups chosen, the Orchidaceae. An appendix provides an account of orchid species in Wallonie, summarizing the history of occurrence of the group in the region.