Although individual electronic tagging using passive integrated acoustic (PIT) tags is established, it is mainly for fish >60. mm in length and is unsuitable for fish of <30. mm, like zebrafish. We used radio frequency identification (RFID) microtags (1 mm in diameter and 6 mm in length, with a mass of similar to 10 mg) to individually identify juvenile zebrafish (length 16-42 mm, mass 138-776 mg) for the first time, and studied the effects of intracoelomic implantation on fish survival and microtag loss, growth, spawning and exploratory behaviour. After 5.5 months, both high survival (82%) and low microtag loss (11%) were achieved. The smallest surviving fish weighed 178 mg, and success in microtag reading was 73% for the size class 350-450 mg (26 mm). Greater success was achieved when fish were larger at the time of tagging but no negative effects on growth were observed for any size class and some tagged fish spawned. No significant differences in behavioural responses could be detected between tagged fish and untagged controls after 2 months. Overall, the results suggest that the tagging method is highly suitable for fish as small as zebrafish juveniles. We think this method will provide significant advances for researchers of the ever-growing fish model community and more generally for all small-fish users. Tagging is essential when one needs to identify fish (e. g. particular genotypes with no external cue), to run longitudinal monitoring of individual biological traits (e. g. growth) or to repeat assays with the same individual at discrete points in time (e. g. behaviour studies). Such a method will find applications in physiology, genetics, behaviour and (eco)toxicology fields.