Although intense fears have been reported in up to 64% of children with an autism spectrum disorder (ASD), little is known about the phenomenology of fear in this population. This study assessed the relationship between fear and core symptoms of autism in children with an ASD. In Phase I of this study, parents of 41 children with an ASD completed the "PDD Behavior Inventory (PDDBI)" and a survey of their child's fears. Social communication abilities were positively correlated with overall levels of fear in children with an ASD. In Phase II the correspondence between parent report of children's fear and direct systematic observations of children's fear reactions was assessed. Results showed poor correspondence between the two assessment methods. Implications of both assessment methods are discussed as well as limitations. (Contains 3 tables and 1 figure.)