The ontogenetic development of size heterogeneity and aggression was monitored in commercial culture tanks of yellowtail kingfish (Seriola lalandi Valenciennes). Size heterogeneity increased substantially with the introduction of Artemia as a food source at 12 days post-hatch (DPH), and was correlated with the appearance of aiming behaviour, a precursor to more direct aggressive interactions. Chasing behaviour started at approximately 19 DPH (10-12 mm total length), with the main aggressors being large-grade individuals comprising 8% of the population. At any one time, only 10-30% of this grade carried out all of the chases, meaning that a very small proportion of the entire population (1%) was responsible for most of the aggressive interactions. The main recipients of aggression were the small grade, comprising 42% of the population, while the medium grade (50% of the population) were generally not aggressive and received only a low to moderate level of aggression. A grading trial showed that large-grade juveniles only displayed aggressive behaviour in the presence of size heterogeneity, and that medium- or small-grade juveniles did not establish an agonistic hierarchy in the absence of large individuals. A high level of aggression in the ungraded control treatment was associated with mortality of most of the small individuals. Even in the absence of aggression, the small-grade juveniles failed to gain weight or show an increase in the RNA/DNA ratio after 12 days. The large- and medium-grade larvae showed an isometric increase in RNA/DNA ratio during development, indicating that faster-growing individuals are likely the result of better food capture or processing traits rather than better protein synthesis rates. Decreasing size heterogeneity and aggression via grading mostly benefits the medium-grade individuals, as the majority of small individuals within a batch appear to be on a degenerative developmental trajectory irrespective of an aggressive environment. 15-20 individuals measured for total length or weight (Less)