The greater noctule (Nyctalus lasiopterus) is one of the rarest, least studied and the largest European bat. It feeds on large flying insects and also preys extensively on small night-migrating songbirds that it presumably intercepts at high altitude. The present study provides the first systematic account of echolocation behaviour and call design in the greater noctule. We documented variability in call duration from 1.4 to 25 ms and an associated strong variation in frequency parameters of echolocation calls. Greater noctules adapted call structure and repetition rate to the current perceptual task. In open space, they used calls with very low peak and terminal frequencies; generally below 15 kHz. We recorded echolocation calls from the sympatric — at least in Eastern Europe — smaller congener N. noctula. Our data suggest that acoustic separation of the two species is feasible. Therefore, acoustic monitoring could prove useful to survey distribution and habitat use of the enigmatic greater noctule. Using still lower call frequency than N. lasiopterus would yield larger detection ranges for large echolocation targets such as migratory birds for the greater noctule, while it might be constrained by the upper hearing threshold of passerines and thus the predator's potential conspicuousness to its prey. However, in a within-genus comparison, allometric scaling explained the greater noctules' call frequency to a very large extent, i.e., we found no evidence for specific adaptations of call frequency to bird hunting.