A new method for tracking small fishes in shallow streams based on passive integrated transponder (PIT) technology, using a portable reading unit, was investigated. The device consists of a chest-mounted palmtop computer, a reader, and a 12-V battery enclosed in a backpack, connected to a 60-cm-diameter coil antenna mounted on a 4-m-long pole. The method was field tested with wild Atlantic salmon, Salmo salar, parr using transponders 23.1 mm long and 3.9 mm in diameter surgically implanted in the peritoneal cavity of the fish. Laboratory experiments indicated no posttagging mortality for fish > 84 mm in fork length and no tag loss when sutures were used. In the field, tag detection distance was up to 1 m. While moving the antenna above the stream surface, the operator could locate a fish's position to within a square metre. Experiments indicated that more than 80% of tagged parr, on average, were detected by the reader. The technique is a useful alternative to standard radiotelemetry in small-scale environments because PIT tags can be implanted in smaller-bodied fishes and fine-scale movements of individuals can be studied. It can be applied to address numerous questions in the fields of animal behaviour, habitat use, and population dynamics.