Selective mutism (SM), considered an early-onset variant of social anxiety disorder, shares features of impaired social interaction and communication with autism spectrum disorders (ASDs) suggesting a possible shared pathophysiology. We examined association of a susceptibility gene, contactin-associated protein-like 2 (CNTNAP2), for ASDs and specific language impairment with SM and social anxiety-related traits.Sample 1 subjects were 99 nuclear families including 106 children with SM. Sample 2 subjects were young adults who completed measures of social interactional anxiety (n = 1028) and childhood behavioral inhibition (n = 920). Five single nucleotide polymorphisms in CNTNAP2 (including rs7794745 and rs2710102, previously associated with ASDs) were genotyped.Analyses revealed nominal significance (p = .018) for association of SM with rs2710102, which, with rs6944808, was part of a common haplotype associated with SM (permutation p = .022). Adjusting for sex and ancestral proportion, each copy of the rs2710102*a risk allele in the young adults was associated with increased odds of being >1 SD above the mean on the Social Interactional Anxiety Scale (odds ratio = 1.33, p = .015) and Retrospective Self-Report of Inhibition (odds ratio = 1.40, p = .010).Although association was found with rs2710102, the risk allele (a) for the traits studied here is the nonrisk allele for ASD and specific language impairment. These findings suggest a partially shared etiology between ASDs and SM and raise questions about which aspects of these syndromes are potentially influenced by CNTNAP2 and mechanism(s) by which these influences may be conveyed.