Complexins are presynaptic proteins that modulate neurotransmitter release. Abnormal expression of complexin 1 (Cplx1) is seen in several neurodegenerative and psychiatric disorders in which disturbed social behaviour is commonplace. These include Parkinsons's disease, Alzheimer's disease, schizophrenia, major depressive illness and bipolar disorder. We wondered whether changes in Cplx1 expression contribute to the psychiatric components of the diseases in which Cplx1 is dysregulated. To investigate this, we examined the cognitive and social behaviours of complexin 1 knockout mice (Cplx1(-/-)) mice. Cplx1(-/-) mice have a profound ataxia that limits their ability to perform co-ordinated motor tasks. Nevertheless, when we taught juvenile Cplx1(-/-) mice to swim, they showed no evidence of cognitive impairment in the two-choice swim tank. In contrast, although olfactory discrimination in Cplx1(-/-) mice was normal, Cplx1(-/-) mice failed in the social transmission of food preference task, another cognitive paradigm. This was due to abnormal social interactions rather than cognitive impairments, increased anxiety or neophobia. When we tested social behaviour directly, Cplx1(-/-) mice failed to demonstrate a preference for social novelty. Further, in a resident-intruder paradigm, male Cplx1(-/-) mice failed to show the aggressive behaviour that is typical of wild-type males towards an intruder mouse. Together our results show that in addition to the severe motor and exploratory deficits already described, Cplx1(-/-) mice have pronounced deficits in social behaviours. Abnormalities in complexin 1 levels in the brain may therefore contribute to the psycho-social aspects of human diseases in which this protein is dysregulated.