| Summary: | The advertisement calls of Nathusius’ pipistrelle (Pipistrellus nathusii) show an unified structure: three main motifs which can be supplemented with additional two motifs especially during mating season. In May and June, a continuous social vocalization of quite unusual structure and composition was recorded apart from these standard calls. Most of these calls were designed in a long “wavy line” pattern with the peak frequency of 14–36 kHz, and were accompanied by standard or modified echolocation calls. In one case we succeeded to supplement the acoustic records with a complete record of behavioural details: two bats slowly flew and followed one another in a circular path in front of the roost, where they repeatedly performed false landings, and during approaching one of them emitted the unusual social calls. This behaviour lasted for about 40% of the whole-night activity. The possible meaning is discussed.
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