Usefulness of Passive Integrated Transponder (PIT tag) technology for observation of behaviour and life history of endangered Unio crassus was tested in two Polish mountain rivers. Dispersion of PIT marked individuals from the place of release did not exceed 3 m. The detection of marked individuals on the rocky bottom was very low (13–39%) and decreasing with time. Ca. 1/3 of implanted PIT tags were rejected, usually within two weeks after implanting; later the rejection did not occur as the PIT tag became fixed in the nacre (very thin on the PIT tag surface adjacent to the flesh, thick with additional fixing structures adjacent to the shell). Nevertheless, 33 tags were detected after three years, some in live individuals which were more numerous on the soft sediment bank (n=12) than on the rocky bottom (n=6). The influence of electromagnetic field on the detection of PIT tags, the possible causes of the tag rejection and mechanisms of tag retention are discussed. It is suggested that PIT tags could be useful as a method of durable individual marking but less suitable for detecting and/or locating the mussels. Controlling of possible tags rejection is indispensable