The 9q34.3 subtelomeric deletion syndrome is a newly defined mental retardation syndrome, caused by haplo-insufficiency of the euchromatin histone methyltransferase 1 (EHMT1) gene. Patients also have childhood hypotonia, facial dysmorphisms, delay in reaching developmental milestones, and behavioral problems like aggressive outbursts, hypoactivity, or autistic-like features. Male and female heterozygous Ehmt1 knockout mice (Ehmt1(+/-), aged 1-20 months, kept on a C57BL/6J background), were used to investigate whether they mimic the patients behavioral characteristics by comparing their behavior to wildtype littermates. The Ehmt1(+/-) mice revealed reduced activity and exploration, with increased anxiety compared to wildtype mice when exposed to novel environments in the open field, object exploration, marble burying, light-dark box, mirrored chamber and T-maze tests. They also demonstrated diminished social play when encountering a mouse from a different litter, and a delayed or absent response to social novelty when exposed to a stranger mouse. However, no differences in phenotyper home cage locomotor activity or rotarod motor function were observed between Ehmt1(+/-) and wildtype mice. Together, these results indicate that the hypoactivity and the autistic-like features of 9q34.3 subtelomeric deletion syndrome patients are recapitulated in this Ehmt1(+/-) mouse model, and that the hypoactivity is apparently not caused by any motor dysfunction. Together, these observations make it plausible that the Ehmt1(+/-) mouse is a faithful mammalian model for the autistic-like behavioral features of patients with the 9q34.3 subtelomeric deletion syndrome.