Transport infrastructure is a critical threat to populations of animals; wildlife crossing structures are therefore commonly applied as a measure to sustain animal movements across roads and railways. Research on the efficiency of crossing structures is needed to provide scientific underpinnings to conservation efforts. Year-round monitoring of a large underpass on the S69 expressway in the western Polish Carpathians revealed that the crossing structure was mainly used by wild mammals (68.6 %), followed by domestic animals (22.1 %) and people (9.3 %). Wild mammals (14 species) were represented by both game and protected species, including largebodied species like red deer Cervus elaphus, roe deer Capreolus capreolus and wild boar Sus scrofa. The Shannon species diversity index and Shannon's evenness index varied seasonally, with the highest number of species recorded in summer and lowest in winter. Red deer neither avoided nor selected for the wildlife crossing structure, while roe deer selected for it, and wild boar avoided it. This study indicates that large underpasses in mountains are used by a rich community of mammals, even if located close to human settlements.