Background: Children with intellectual disability are at risk for anxiety disorders involving intense physiological reactions and risky behavioral responses. Interventions have been identified in this field; however, assessment of underlying anxiety is limited and flawed.Method: We implemented a single-subject case study using differential reinforcement to treat dog phobia in a boy with intellectual disability. We recorded elopement and compliance with goals and measured physiological expressions of stress: galvanic skin response, heart rate variability, temperature, and latency to calm down.Results: After fifteen therapy sessions, the boy decreased elopement and noncompliance considerably and showed dramatic improvements in emotional self-regulation.Conclusions: Future research should examine the utility of including biosensing measures in clinical applications and the relationship between physiological measures of anxiety and traditional questionnaires. Children with intellectual disability at risk for anxiety disorders should be tracked longitudinally to examine the effect of interventions on social-emotional well-being and self-regulation.