The Eurasian Bittern (Botaurus stellaris) is a characteristic bird of extended reed beds. Most easily the presence of bittern can be detected by their characteristic booming vocalizations. A booming sequence consists of a series of tonal calls with a fundamental frequency of approximately 150 Hz. The calls could be heard over distances of more than 1 km, but despite of their low frequency the exact position of the caller could be hardly estimated by an observer. In addition in areas with high population density it makes difficulties to discriminate individual birds by the acoustic characteristics of their calls. We assessed the number of booming bitterns in a large wetland area in the Northeastern part of Germany. A set of four-channel-recorders was used to record the vocalizations of the birds. The synchronization of the recording units was realized by recording a radio signal at each recorder at the beginning of the recording session. The position of callers was estimated by the directional information of each four-channel-recorder and the differences in the time of arrival of the acoustic signal at the different recording devices. Time differences were calculated using cross correlation techniques. The method allowed to distinguish between at least 14 booming bitterns in a reed bed area of only two square kilometres. The main advantages of the acoustic triangulation are that the method could be applied to large areas and the data acquisition could be realized by a single observer. The study was supported by the Deutsche Bundesstiftung Umwelt.