Selective serotonin reuptake inhibitors (SSRIs) are psychotropic pharmaceuticals used as antidepressants. SSRIs are commonly found in surface waters in populated areas across the globe. They exert their efect by blocking the serotonin re-uptake transporter in the presynaptic nerve ending. The present study examined whether behavioural efects to exposure to SSRI citalopram depend on personality and sex in the stickleback (Gasterosteus aculeatus). Three aspects of stickleback behaviour are examined: feeding behaviour, aggression, and boldness. We exposed sticklebacks to 350–380 ng/l citalopram for 3 weeks. Feeding and aggressive behaviour were recorded before and after exposure, whereas scototaxis behaviour was tested after exposure. The results show treatment efects in feeding and aggressive behaviour. Feeding is suppressed only in the male group (χ2=20.4, P<0.001) but not in the females (χ2=0.91, P=0.339). Aggressive behaviour was signifcantly afected by treatment (χ2=161.9, P<0.001), sex (χ2=86.3, P<0.001), and baseline value (χ2=58.8, P<0.001). Aggressiveness was suppressed by citalopram treatment. In addition, the fsh showed no change in aggression and feeding behaviour over time regardless of sex and treatment, which indicate personality traits. Only females are afected by treatment in the scototaxis test. The exposed females spent signifcantly (χ2=5.02, P=0.050) less time in the white zone than the female controls.