Discrimination of quantity (magnitude) was investigated in zebrafish (Danio rerio). Male zebrafish chose to approach the location previously occupied by the larger in number between 2 groups of female conspecifics (no longer visible at test) in sets of 1 versus 2 items, and 2 versus 3 items, but failed at 3 versus 4 items; similarly, when tested with larger numbers, zebrafish succeeded with 2 versus 4, 4 versus 6, and 4 versus 8 items, but failed with 6 versus 8 items. The results suggest that zebrafish rely on an approximate number system to discriminate memorized sets of conspecifics of different magnitudes, the degree of precision in recall being mainly dependent on the ratio between the sets to be discriminated.