The karstic area between the rivers Timok and Vit has about 500 caves and pot-holes, more than 300 of them being the subject of the present study. In this area (called here Western Stara Planina) we have noted 63 species and subspecies of terrestrial troglobites and 17 species and subspecies of aquatic troglobites. The two Regions into which we split the area (region of Ogosta and Region of Iskar) are the richest in cave fauna of Bulgaria. This study deals with the particularities in the composition and the distribution of the different groups of terrestrial troglobites, and especially of such groups as Diplopoda, Isopoda, Coleoptera and others. According to the paleogeographic development of the area conclusions are made concerning the age of certain troglobites and the barriers determining their present distribution.