Field observations of the different physiological colour patterns (PCPs) of juvenile bullethead parrotfish,Chlorurus sordiduswere conducted at three sites around Lizard Island, Australia. The aim of the study was to quantify, using log-linear analysis, the influence of simultaneously interacting variables upon the PCPs shown by this species in the field. It was proposed that features of an individual, such as body size (total length) and schooling behaviour (schooling/not schooling), as well as foraging activity (feeding/not feeding), were influential in PCP determination. In addition, the influence of time of day was considered. Overall, an individual's body size was the most significant factor, being over six times more important than foraging activity in determining the PCP displayed. The schooling behaviour of an individual was 40% more important than foraging activity in PCP determination. Furthermore, the correlation between body size and PCP was itself dependent on time of day whilst both foraging activity and schooling behaviour were independent of time of day. The systematic methodology used enabled interactions between variables to be ranked numerically and their biological significance to be discussed. The results present correlational evidence that PCPs may function for anti-predator purposes and provide the basis for future studies into the function of PCPs in juvenileC.sordidus