Anxiety and depressive symptoms commonly co-occur yet the underlying mechanisms for this covariation remain poorly understood. Genetic strategies are a useful means of investigating whether the comorbidity of two sets of symptoms or disorders can be explained by the same aetiological factors. In this paper we use a systematically ascertained sample of 172 twin pairs aged 8 to 16 years to examine the causes of covariation of maternally rated anxiety and depressive symptoms. The results suggest that most of the covariation can be explained by a common set of genes that influence anxiety and depressive symptoms. Some covariation between anxiety and depressive symptoms is also explained by environmental influences of the non-shared type. In addition, depressive symptoms also appear to be influenced by specific genetic factors.