The process of translocation of endangered species represents a useful conservation tool, but subsequent monitoring of translocated populations is often neglected, although it may supply critical information for conservation work. The EU-protected glacial relic Lycaena helle (Lycaenidae) went extinct in the Czech Republic, where a few lowland populations historically existed, in the 1950s. In 2002, a mountain-dwelling population was established in Šumava Mts., among grasslands and fenlands surrounding the abandoned village of Nové Údolí, from 2/33 ♂♂/♀♀ originating in the Türnitz Alps, Austria. The transferred population was surveyed a decade later and again 21 years after the transfer. During the second (2023) survey, we visited 212 grassland patches to record the species’ presence and relative abundance, and to ascertain its habitat preferences. A third of the surveyed patches were occupied by L. helle, in a total area of occupied grasslands was 66.6 ha scattered over 4.5 km2, with approximately 2,400 adult individuals, recorded up to 4.2 km from the original release point. L. helle typically occupies wet tussocky grasslands with closed edges, minimum management and high cover of its larval host plant, the mid-to late-successional forb Bistorta officinalis. It avoids drier managed grasslands rich in flowering plants. The expansion rate from the release point (0.1 km/year during the first decade, 0.2 km/year over two decades) was slower than in two earlier L. helle transfers, targeting Morvan and Forez Mts., France (≈ 0.5 km/year in both cases). We attribute this to the relief of the release locality, a­ shallow depression surrounded by higher-elevated forested ridges, but we expect faster expansion once the continuous woodlands are crossed.