Osmia pilicornis is distributed from western temperate Europe to western Siberia, where it exclusively occurs in open-structured, mesophilous and mainly deciduous woodland below 1000 m a.s.l. In Central Europe, its peak activity ranges from the last third of March to the first third of June. Due to its rarity and its low population densities over most of its range, the biology of O. pilicornis was only fragmentarily known. The discovery of six nests in the course of the present study revealed that females of O. pilicornis have a unique nesting behaviour among the osmiine bees: they gnaw their nests in dead wood with the aid of their strong mandibles, which have a peculiar chisel-like shape hypothesized to be an adaptation to the species’ specialized nesting behaviour. All six nests were in dead fallen branches of different tree and shrub species and of varying wood hardness. The nesting branches had a diameter of 1.5–6.1 cm, lay on sun-exposed ground and were largely hidden under vegetation. The nests contained one to three linearly arranged brood cells. Both cell partitions and nest plug were built from chewed leaves harvested from Fragaria vesca. Osmia pilicornis was identified as a new host of the chrysidid wasp Chrysura hirsuta, and the ichneumonid wasp Hoplocryptus confector developed in its nests. Microscopical analysis of scopal pollen loads of collected females revealed that pollen is mainly collected from three plant taxa, i.e. Pulmonaria (Boraginaceae), Fabaceae (e.g. Lathyrus, Vicia) and Lamiaceae (e.g. Ajuga, Glechoma). On flowers of Pulmonaria, which is the most important pollen host over most of the species’ range, the females use specialized bristles on their proboscis to brush pollen out of the narrow corolla tube, they almost exclusively exploit pollen-rich flowers in the early red stage and they often steal pollen from still closed flowers by forcefully opening buds. On their search for females, males of O. pilicornis patrol flowers of Pulmonaria in a rapid flight regularly interrupted by short resting periods on the ground. Females are grasped for copulation both during flower visits and in flight between the flowers. The wide spectrum of semi-open mesophilous woodland types colonized by O. pilicornis suggests that dead fallen branches and a rich spring flora in combination with a rather warm but not xeric microclimate are the only requisites needed by the species. As the great majority of woodland habitats currently occupied by O. pilicornis in Central Europe owe their origin to human forest use, the recent decline of O. pilicornis in many regions of Europe may have been caused by changes in woodland management practices leading to closed and dark forests not suitable as habitats for this specialized bee species.