To determine whether aberrant behaviors described in boys with fragile X syndrome distinguish them from other boys with developmental disabilities, we asked the primary caregivers of 55 boys with fragile X and 57 matched controls to complete five behavioral questionnaires. Twenty-one items distinguished the groups. Principal components analysis (PCA) yielded five behavioral clusters: abnormal language, tactile defensiveness, poor self-control, poor eye contact/shyness, and hand flapping. Boys with fragile X were found times more likely to have both tactile defensiveness and abnormal language. The presence of these abnormal behaviors in boys with developmental delays warrants further assessment for fragile X.