This article reports on part of a commissioned research study into students with severe challenging behaviour in primary schools serviced by the Catholic Education Office (Parramatta Diocese) in western Sydney. The focus of the study was on the prevalence of severe challenging behaviour and the nature of presenting behaviour. Questionnaires were directed to school staff and information was obtained from 41 of the 53 primary schools in the diocese. Using very conservative criteria, the estimate of numbers of students with severe challenging behaviour was approximately 1 per school. Students were typically male and were academically below average. The most frequently reported challenging behaviour (e.g., calling out, out of seat) was inherently minor in nature for the most part, but at high frequency this could be extremely disruptive to the operation of a classroom. More serious behaviours, such as physical aggression to other school students and staff, were also reported at concerning frequency, noting that such behaviours place staff and other students at risk. The present study adds to the limited Australian data describing students in regular schools with challenging behaviour by providing specific information on the classroom frequency of such behaviour and the academic performance of students.