There are two methods of ultrasound acquisition and analysis with PC's: The first and most straightforward solution is a special data acquisition board with a sampling frequency high enough to capture the entire bandwidth of the signals to be analysed. This requires taking the computer into the field if you have to do field research. The best choice would be a PCMCIA data acquisition card for portable notebook PC's. Unfortunately the devices available on the market still have limited sampling frequencies of approximately 100 kHz. This allows a usable bandwidth of less than 50kHz, which is not sufficient for all kinds of ultrasound. The data streams generated by these data acquisition boards can be stored on hard-disk and can then be read by the Avisoft-SONAGRAPH software using one of its user-defined import formats. The alternative method is the usage of a digital time-expansion bat- detector to transform the ultrasound into low frequency sound which can be processed by conventional audio equipment. These transformed sounds can be stored on standard tape recorders and can be transferred into the computer using a common sound-card. In this case the transformed ultrasound is treated like any other audio signal. In order to get the original scaling of waveforms, spectra and sonagram displays, the time-expansion factor used on the bat-detector can be specified in the Avisoft -SONAGRAPH software. Conventional bat-detectors using heterodyne or frequency division technology are not suited for spectral analysis on a computer. These devices could only be used to get an idea of the time structure of the signals.