The interest for commercial rearing of Atlantic cod (Gadus morhua) has grown over the last decades. However, heterogeneous growth and cannibalism during early juvenile stages causes serious financial losses. Generally in fishes, dominance based hierarchies leads to heterogeneous growth. Furthermore, a close relationship between cannibalism and heterogeneous growth has been observed. Moreover, it has been demonstrated that an increase in serotonergic activity is assosiated with suppressed aggressive behaviour in various vertebrate species. For instance, dietary feed supplemented with the serotonin precursor tryptophan (TRP) has been shown to suppress aggressive behaviour in rainbow trout. However, little is known about the social organization, aggressive behaviour and the the underlying neural mechanisms of intraspesific competition in juvenile cod. To develope a method for quantifying aggressive behaviour in juvenile cod, two different protocols were tested, and the effects of dietary TRP supplementation on aggression was investigated. The study also included an analasys of the effects of dietary TRP supplementation on brain serotonergic activity (indexed by the ratio between the serotonin metabolite 5-hydroxyindoleacetic acid, and serotonin). Following an initial control period, fish were given TRP supplemented feed for seven days. This resulted in a decrease in aggressive behaviour when it was quantified with daily pairwise interactions (experiment 1). Furthermore, TRP treatment resulted in an increase in serootnergic activity (experiment 2). However, when aggression was quantified by a resident-intruder test protocol (experiment 2), no effects on aggression of dietary TRP supplementation was observed. The absence of a measurable suppression of aggression in experiment 2 could be related to stress induced by the resident-intruder test, suggesting that this protocol is less suitable for detecting changes in aggressive behaviour in juvenile cod. In conclusion, this study shows that juvenile cod are highly aggressive, and that dietary TRP supplementation can suppress this behaviour. Thus, TRP supplementation may offer a presumptive strategy for decreasing aggression and associated problems, such as size heterogeneity and cannibalism, in cod rearing.