The zebrafish leopard phenotype (leo) displays abnormal pigmentation and shows increased anxiety-like behavior. The neurochemical changes associated with this anxious phenotype are not known. Here, we demonstrate that leo show increased anxiety-like behavior in the light/dark box and in the novel tank test. This anxious phenotype is rescued by acute treatment with a dose of a serotonin reuptake inhibitor, fluoxetine, that is inactive in wild-type animals. Moreover, leo show decreased tissue levels of serotonin, increased serotonin turnover and slightly increased monoamine oxidase activity. These results suggest that the anxious phenotype observed in leo zebrafish is caused by a decrease in serotonin uptake. This work could open an important avenue in defining the neurochemical underpinning of natural variation in anxiety disorders.