ABSTRACT: This study quantifies the schooling behaviour of a coral reef fish and, in particular, examines the stimuli that may motivate school formation. The focus of the study was the parrotfish Chlorurus sordidus (family Scaridae) which, in thejuvenile phase, is able to undergo rapid physiological colour pattern ('PCP') changes. Juvenile C. sordidus frequently schools with other juvenile parrotfish species that share some PCPs. Solitary juveniles were followed in the field before andafter joining a school to investigate whether their behaviour correlated with characteristics of the school joined. The variables recorded were size and PCP of focal juveniles, size and composition of schools and the feeding behaviour and PCPs displayedby the school members. Data were systematically collected and analysed using log-linear modelling to quantitatively examine the relationship between all possible combinations of the variables measured. The results of the study provide correlationalevidence that juvenile C. sordidus display assortative schooling based on PCP and size characteristics of schools. In addition, the foraging behaviour of schools was influential in determining an individual's schooling behaviour. These data serveto highlight the complex and dynamic nature of schooling behaviour in this species.