<jats:title>Abstract</jats:title><jats:p>Neuropeptide Y (NPY) controls energy homeostasis including orexigenic actions in mammalians and non-mammalians. Recently, NPY has attracted attention as a mediator of emotional behaviour and psychosomatic diseases. However, its functions are not fully understood. We established <jats:italic>npy</jats:italic> gene-deficient (NPY-KO) zebrafish (<jats:italic>Danio rerio</jats:italic>) to assess the relationship between NPY and emotional behaviours. The NPY-KO zebrafish exhibited similar growth, but <jats:italic>pomc</jats:italic> and <jats:italic>avp</jats:italic> mRNA levels in the brain were higher as compared to wild-type fish. NPY-KO zebrafish exhibited several anxiety-like behaviours, such as a decrease in social interaction in mirror test and decreased locomotion in black-white test. The acute cold stress-treated NPY-KO zebrafish exhibited anxiety-like behaviours such as remaining stationary and swimming along the side of the tank in the mirror test. Moreover, expression levels of anxiety-associated genes (<jats:italic>orx</jats:italic> and <jats:italic>cck</jats:italic>) and catecholamine production (<jats:italic>gr</jats:italic>, <jats:italic>mr</jats:italic>, <jats:italic>th1</jats:italic> and <jats:italic>th2</jats:italic>) were significantly higher in NPY-KO zebrafish than in wild-type fish. We demonstrated that NPY-KO zebrafish have an anxiety phenotype and a stress-vulnerability like NPY-KO mice, whereby <jats:italic>orx</jats:italic> and/or catecholamine signalling may be involved in the mechanism actions.</jats:p>