Background: The identification of specific nonshared environments responsible for the variance in behaviour problems is a key challenge. Methods: Nonshared environmental influences on teacher-reported behaviour problems were explored independently of genetics using the monozygotic (MZ) twin differences design. Six aspects of classroom environment were rated by a representative sample of 570 nine-year-old MZ twins in the UK in different classrooms and were related to their different teachers' reports of prosocial behaviour, hyperactivity, conduct problems, peer problems and emotional symptoms. Results: Within-pair differences in perceptions of the classroom were significantly correlated with teacher-reported behaviour problems, indicating that children with less favourable perceptions of their classroom environment were reported by their teachers as less prosocial, more hyperactive, and to have more conduct and peer problems. Socioeconomic status did not significantly moderate any of these relationships. However, parent-reported household chaos was a significant moderator. Conclusions: The classroom environment is related to behaviour problems even when genetic factors are held constant. Classroom environment is more strongly associated with behaviour problems when the home environment is more chaotic.