In the past 25 years much attention has been given to the development and use of instruments to assess the qualities of the classroom and school environment from the perspectives of students and teachers. This paper describes the development, validation and application of an instrument, the Principal Interaction Questionnaire, to measure principals' interpersonal behaviour in a school. The new instrument was based on the Questionnaire on Teacher Interaction, first devised in The Netherlands. It contains eight scales to measure various aspects of a principal's interpersonal behaviour such as Directive behaviour, Strict/Inflexible behaviour and Understanding behaviour. The questionnaire was sent to 56 schools throughout Australia where it was completed by the principal and a random sample of 20 teachers in each school. The results obtained were analysed and provided measures of the interpersonal behaviour of principals as perceived by themselves and their teachers.