This paper describes two prototypes of portable detectors based on radio frequency identification modules and antennas commercially available in Europe and North America for reading small (2.1-mm × 11.5-mm) passive integrated transponder tags. Maximum tag detection distances ranged from 17 to 36 cm, depending on the system and orientation of the tag to the antenna. The efficiency of the detectors was field-tested with both wild juvenile brown trout Salmo trutta and adult slimy sculpin Cottus cognatus that had been marked by injection of a tag into the peritoneal cavity. By probing the water with the antenna, we were able to detect, on average, 69% of age-1 trout (fork length, 148 ± 26 mm [mean ± SD]), 82% of age-0 trout (fork length, 72 ± 8 mm), and 82% of adult sculpin (total length, 74 ± 9 mm). We did not conduct a formal study to compare the performances of the two prototypes or to determine how habitat characteristics may alter their efficiency.