We investigated the oviposition preferences of Phengaris alcon in relation to the height of the host plant (Gentiana pneumonanthe), the number of flowers, and the choice of microsite for oviposition (part of the host plant, height of oviposition) in a local population of this species in SW Slovenia at a depression of a seasonal lake Petelinje. P. alcon prefers the tallest (most conspicuous) host plants and plants with a larger number of flowers (i.e., more than 90% of all eggs were deposited on 12% of the tallest plants ≥ 30 cm), where eggs are deposited in the upper quarter of the plant height, usually on calyx (66.1%) and corolla (29.9%), and rarely on leaves or stems. Eggs are mostly deposited singly or in groups of two (> 50%) on a single plant, and eggs were found only on less than 3% of examined plants. Butterfly density was estimated at about 60 individuals per hectare. Obtained results indicate that preferences for oviposition sites are not random, and monitoring population density may provide important insight into the future population status of P. alcon.