The population size of three troglobitic species of Speonomus (Coleoptera Bathysciinae) from the Pyrenees, intentionally introduced in 1969-1970 into the “Grotte de Ramioul” cave (Province of Liège, Belgium) has been studied using the mark-recapture technique. It has been shown that the three cave levels possess large Speonomus populations (upper network: 12,718; middle network: 13,902; lower network: 18,249 individuals) and that the superficial underground environment is also colonized, including the schist area. The most abundant species is Speonomus longicornis (relative frequency between 0.73 to 0.89). The two other species are present in the relative frequency between 0.04 and 0.23 for S. diecki, and between 0.03 and 0.11 for S. stygius. The overall Speonomus population size in the cave is estimated at 44,000 individuals.