Tagging or marking small laboratory-bred fish species is not an easy task. This also holds for the zebrafish, Danio rerio, which is widely used throughout the world as a model organism for genetics, developmental biology, etc. We present a simple marking technique based on scale regeneration. A comparative morphological study of various types of zebrafish scales indeed shows that regenerated scales are easily distinguishable from nonregenerated ones. We propose to take advantage of this typical morphology to mark a single or several individuals. This technique, based on a natural biological process, is easy to perform and does not enhance fish mortality in laboratory breeding conditions. It permits assembly of several specimens in a single tank with the possibility of identifying each of them by regenerated-scale coding. Nevertheless, a prerequisite is that the species does not lose and regenerate scales in large numbers in laboratory breeding conditions. To check this, 5,200 scales were removed from a large region of the left flank in 100 zebrafish and the number and position of regenerated scales were statistically analysed. Our results indicate that (1) laboratory-bred zebrafish have only a few regenerated scales (7.48%), (2) the probability of finding a regenerated scale is similar whatever its position in a row (antero-posterior axis), but (3) it differs from one row to another (scales from the back are more frequently lost than those from the pectoral region). This paper presents a procedure to mark small breeding colonies of zebrafish using scale regeneration with the number and position of the scales to be removed with high probability of marking success. J. Exp. Zool. 286:297-304, 2000.