Research on human infants and non-human animals has demonstrated that rudimentary numerical abilities pre-date the evolution of human language. Nonetheless animals can base their quantity judgments on continuous perceptual cues that correlate with number and it is unclear whether animals other than mammals and birds can use number. In the present work we adopted two different approaches to investigate whether basal vertebrates, such as fish, can discriminate between small quantities by using numerical information only. In experiment 1 we investigated the non numerical perceptual cues preferentially used by fish to discriminate between two and three elements; in experiment 2 fish were then trained to distinguish between the two quantities when continuous variables were controlled for. Lastly (experiment 3) we observed the spontaneous choice between a group of two and a group of three social companions using a method similar to item by item presentation of the stimuli to prevent the access to non-numerical information. On the whole, our results suggest that fish can represent and use numerical information for discriminating small quantities.