Using a self-administered questionnaire, the attitudes of 844 Maltese primary school teachers towards 16 selected undesirable behaviours were surveyed. Results indicated that pupils' age is a significant moderator of half of the behaviours; these are perceived to be significantly more serious by teachers in the second three years of the primary school than by their colleagues in the first three years. Significant boy-girl differences in the seriousness of behaviour were revealed in most of the behaviours as rated by teachers in both the first and in the second years. In addition, length of teaching experience was found to be a significant moderator of the perceived seriousness of some of the behaviours: the less experienced, when compared with the more experienced, judged these behaviours as being more serious. Ability stream did not have any significant effects on the teachers' attitudes towards the undesirable behaviours.