This paper describes a system based on a commercial portable energizer-reader module and antennas and a hand-held programmable calculator for reading large (4 ϫ 23 mm and 4 ϫ 32 mm) passive integrated transponder (PIT) tags. Maximum tag-reading distances in water were 45 cm for ferrite-core stick antennas and 86 cm for a loop (gate) antenna; these distances varied with the size of the tags and their orientation to the antennas. The tags, tested on wild brown trout Salmo trutta and wild rainbow trout Oncorhynchus mykiss, were either inserted into the peritoneal cavity with a hypodermic needle or attached externally at the base of the dorsal fin with a fish hook and monofilament line. The fish were held in artificial channels 1 m wide and 50 m long and in two 3-4 m-wide stream sections, one 70 m long and the other 90 m long. During a 5week period we were routinely able to detect marked fish by (1) walking along the channels and stream sections probing the water with an antenna, and (2) by positioning antennas on the substrate of the channels and recording tags from undisturbed fish as they swam by. We did not conduct a formal study of the long-term suitability of either type of tag placement or of the overall efficiency in detecting marked fish.