Background: Individuals with intellectual and developmental disabilities are more likely to engage in problem behaviours than peers with typical development. The purpose of this study was to provide descriptive and qualitative information about problem behaviours in children with fragile X syndrome (FXS) and how families respond to these behaviours. Method: We examined interview responses from 53 mothers of 9-year-old children with FXS. Results: Defiance, tantrums, inattention, stereotypy, and aggression were the most frequently reported problem behaviours of children with FXS. Stereotypy, physical aggression, self-injury, and elopement were reported more often by mothers of children with dual diagnoses of FXS and autism than by mothers of children with a single diagnosis of FXS. Conclusions: Results indicated that for many families, by 9 years of age, problem behaviours led to frustration and altered family activities.