In dolphins, directional, broad-band pulsed sounds are used for echolocation but due to their high directionality, they are also well suited for addressing intra-specific, social signals. Studies on pulsed communication sounds in free swimming dolphins have so far been restricted to the audible range, not dealing with the directional aspect. Preliminary studies at our facility indicate that dolphin use this option in their intra- specific communication. We have developed a new Pc-compatible datalogger concept for recording and store high frequency pulsed sounds produced by free swimming bottlenose dolphins Tursiops truncatus. The recording unit, called 'MOSART' (MObile Subsurface Acoustic Recording of Transients), is designed to record the highly directional high frequency sounds received by a dolphin, not those produced by it. The 'MOSART' will be attached to the dorsal in by means of a non- invasive saddle pack (Trac Pac). The 'MOSART' has a broad-band hydrophone, and specially developed sound processing electronics and software. The sounds are stored on a 1.8 inch hard disk. Several recording options will be adjustable by means of computer software settings, (e.g. time delay before recording onset, trigger amplitude threshold, high pass filter limit and sampling rate). To maximise the recording time, only a limited and pre-set number of fully sampled clicks will be collected. For the rest of the clicks only a time stamp will be stored. In this way at least 86 minutes of continues click trains, with an average repetition rate of 500 pps, may be recorded. The stored data will be transferred to a PC for analysis. It will be compared with traditional broad band recordings with fixed hydrophones and correlated with video recordings of the dolphin behaviour.