A research project on the conservation biology of the rare Apium repens in Belgium is presented. The species is still present in five localities, whereas it has certainly occurred in at least 24 localities in the past. The (aut)ecology (soil nutrient levels and hydrology) and phytosociology of the species are studied within the Belgian populations. Presence/absence monitoring is carried out as well as demographic monitoring in order to determine the trend in population size, the plant density, and the number of flowers and of seeds produced per plant, and to relate these to soil water level and grazing pressure. Methods are given in some detail. Results show that the surface area of four localities in which the species grows ranges from 5 m2 to 6 ha. Within these areas A. repens is present with average densities ranging between 1 and 200 ramets per m2. Locally, densities up to more than 2200 ramets per m2 were found in frequently and short cut lawns of a public park. The disturbance effected by the mowing appears to offer optimal conditions, leading to plant densities that equal or surpass those in naturally optimal conditions. In another part of the investigation, uncertain taxa are identified by molecular techniques and the genetic variation within and between the populations is measured.