Background: Identifying many of the diagnostic criteria for anxiety and depression in individuals with intellectual disability (ID) can be challenging because they may be unable to recognize and communicate their emotional experiences accurately. The purpose of this study is to identify behavioural equivalents of anxiety in children with fragile X syndrome (FXS), the leading inherited cause of ID. Methods: Parents and teachers of 43 children (aged 6-14 years) with full mutation FXS completed two standardized questionnaires on children's problem behaviour and psychiatric symptoms. Items from the questionnaires thought to be possible behavioural equivalents of anxiety were identified and grouped into four domains: Avoidance Behaviours-Confrontational; Avoidance Behaviours-Non-confrontational; Anxiety Continuum Behaviours; and Behavioural Dysregulation. The mean rating for the four groups of items was used to predict the children's status for exhibiting significant problems with anxiety as defined by the Diagnostic and Statistical Manual of Mental Disorders-oriented Anxiety Subscale from the problem behaviour scale. Results: The predictor variables classified 81% (parent rating) and 86% (teacher rating) of the children correctly. Avoidance Behaviours - Confrontational and Avoidance Behaviours - Non-confrontational (teacher rating) and Anxiety Continuum Behaviours (parent and teacher rating) made unique contributions to the models. Conclusions: Children who are unable to identify and communicate that they worry about general day-to-day events may exhibit more observable behaviours resembling active and passive avoidance (e.g. arguing, avoiding difficult tasks, staring off) or have specific phobias and compulsions. These findings suggest that there are behavioural equivalents for anxiety disorder in children with FXS and, more generally, support the notion of behavioural equivalents in ID.