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Majority of keys for identification of the European bats in a field suggests to discriminate Pipistrellus nathusii from Pipistrellus pipistrellus according to different frequencies of their ultrasound signals that at amplitude maximum are 38-39 kHz for P. nathusii and 45 kHz for P. pipistrellus. The present study was designed to determine the variability of frequency at amplitude maximum (FAM) in the ultrasounds of P. nathusii which is common and widespread bat species in Lithuania.
 
Data collection was performed in the western and central parts of Lithuania in May-July 2001, in the vicinity of four maternity colonies in which the absolute predominance of P. nathusii was confirmed by a mist netting of bats in front of their roosts. For a reliable recording of ultrasounds from P. nathusii, ten animals of this species were labeled with fluorescent tags that were made from small plastic tubes (20 mm in length, 2,5 mm in diameter) and stuck on animal dorsal hairs for using a nontoxic dry paper glue. The bat ultrasounds were detected by means of a time expansion detector (Pettersson Elektronik AB, model D-240) and recorded on minidiscs with a portable minidisc recorder (Sony, model MZ-R700). The recordings were analyzed on a computer using a BatSound software (Pettersson Elektronik AB, version 1.3.1).
 
Analysis of 150 sequences (lasting up to 1,7 seconds) of ultrasound signals recorded during various animal flies (prey searching, display fly, etc.) near to bat roosts demonstrates that in all study areas P. nathusii emitted ultrasound signals with FAM ranging from 38 up to 47 kHz. Prevalently, the P. nathusii signals recorded in Lithuania, including those from animals that were labeled with fluorescent tags, varied between 41 and 43 kHz. 
 
The present study evidently revealed that in Lithuania P. nathusii can not be discriminated from Pipistrellus pipistrellus basing exclusively on the exactly determined values of FAM. Until other discriminating particularities of the P. nathusii ultrasound signals are not confirmed and/or determined, no discrimination should be done between P. nathusii and P. pipistrellus distributed in Lithuania.