The validity of using radio-telemetry, specifically variation in radio-signal strength, to quantify changes in activity of radio-tagged aquatic fauna was investigated under controlled conditions. We monitored short-term activity of two aquatic species, rainbow trout (Oncorhynchus mykiss) and Murray River crayfish (Euastacus armatus), that differ markedly in their primary mode of movement. Records of variation in signal strength were compared with the frequency, duration and type of movements as confirmed by time synchronised video. Active and inactive periods for both species were accurately detected by radio-telemetry. However, determining specific types of movement (i.e. burst swimming, cruising etc) proved problematic. Our findings are used to discuss applications of this technique for monitoring activity of site attached species.