During a three-year tracking study on the river Tywi in south-west Wales, over 400 adult salmonids were fitted with SAL3 tags of MAFF design. In all years of study, a proportion of fish were undetected post-tagging or disappeared during the course of tracking. In order to eliminate or identify tag failure as the cause of 'losses,' control experiments were established, with tags drawn from the batches used for fieldwork. This chapter briefly describes results from these control experiments.