Acoustic signals are the primary medium for communication. Nevertheless, the diversity of bat vocal repertoires remains poorly known. Recently, several studies have demonstrated that males of some species produce highly versatile songs during courtship, suggesting that this still poorly understood phenomenon may also occur in other species. We investigated the courtship vocalisations of 28 individuals of the lesser mouse-eared bat Myotis blythii in five underground sites in southern Ukraine (Eastern Europe). We analysed the spectrographic structure and syntax of nearly 9,000 bat vocalisations. We identified nine syllable types forming phrases, and different phrases were combined into vocalisation units (hereafter referred to as vocalisations). The number of syllables within phrases and the number and order of phrases within vocalisations were highly variable. We identified 689 phrase orderings (vocalisation types) and 1,423 variants resulting from variable repetition of syllables within phrases (vocalisation variants), of which only 20 vocalisation types and 19 vocalisation variants accounted for more than 1% of the total analysed repertoire. Transition structure was weak (i.e., the frequency with which one phase was followed by another), as no single phrase transition accounted for more than 9% of all adjacent transitions. Phrase types also exhibited only weak positional biases within vocalisation units, indicating limited sequential structuring. These results suggest a low level of stereotypy and a dynamic expansion of courtship vocal display through phrase reassertion and variable repetition of a limited number of syllables. This strategy makes the courtship vocal repertoire of M. blythii one of the most diverse vocal systems described in mammals. Given its structural complexity and its production by males during the mating season at courtship sites, the analysed vocal repertoire qualifies as song.