The authors provide an outline of actual knowledge on the distribution of the Pygmy Owl (Glaucidium passerinum) in Bulgaria from 1900 until 2008. The most recent estimates of the species population in Bulgaria are 150-200 and in Slovakia 1500-2000 breeding pairs. On the basis of quantitative samples obtained in the Rila Mts (Bulgaria) and in the High and Western Tatra Mts, Low Tatra Mts, "Krivánska Malá Fatra" Mts, "Veľká Fatra" Mts, "Chočské vrchy" Mts, "Stolické vrchy" Mts (Slovakia) obtained densities in Bulgaria are compared with ascertained densities in similar elevations in Slovakia. In the Rila Mts, 9 territories were found / 23 km2 and average density 0.39 territories / 1 km2 was found, in 1350-2000 m elevation. In Slovakia 75-82 territories were found / 79 km2 and average density 0.95-1.04 territories / 1 km2 was found, in 800-1650 m elevation. The density of Pygmy Owls in the mountains of Bulgaria is twice as low as in the mountains of Slovakia in Western Carpathians. This is probably caused by the distribution at the edge of the distribution area of the species and an higher isolation level in the Balkans. Other evaluated factors, such as habitat quality, health of forest stands, shorter twilight and shorter owl activity in the Balkans than in the Carpathians, negative effect of streams on voice activity of the Pygmy Owls, lack of food supply or lack of breeding holes, or even possible competition with other owl species were not regarded as reasons causing differences in Pygmy Owl densities in compared areas.
