(Uploaded by Plazi for the Bat Literature Project) Since 1978 we have used ultrasound detectors for field studies of European bat species and large scale mapping and monitoring in Denmark and Sweden. The method has revolutionized the field studies of bats with great possibilities and advantages. Most of the 31-32 European bat species can be identified with bat detectors, but in practical work a few species pairs may have to be lumped e.g. Myotis mystacinus/brandtii. The species are not equally easy to find and identify, and some may need considerable time to be identified. No single variable of bat sound can be used to separate all species, and identification is often based on a number of characters in combination. Both acoustic and visual clues are of importance. Analyses of recorded sounds are valuable but do not stand alone; it is important to gain as much information as possible on the spot from the total situation in the field. We use ultrasound detectors equipped with heterodyne and time expansion systems in combination. This combination has many advantages for instant identification as well as subsequent analysis.