Social interactions in small groups of juvenile rainbow trout (Oncorhynchus mykiss) lead to the formation of dominance hierarchies. Dominant fish hold better positions in the environment, gain a larger share of the available food and exhibit aggression towards fish lower in the hierarchy. By contrast, subordinate fish exhibit behavioural inhibition, including reduced activity and feeding. The behavioural characteristics associated with social status are likely the result of changes in brain monoamines resulting from social interactions. Whereas substantial physiological benefits, including higher growth rates and condition factor, are experienced by dominant trout, low social status appears to be a chronic stress, as indicated by sustained elevation of circulating cortisol concentrations in subordinate fish. High cortisol levels, in turn, may be responsible for many of the deleterious physiological consequences of low social status, including lower growth rates and condition factor, immunosuppression and increased mortality. Circulating cortisol levels may also be a factor in determining the outcome of social interactions in pairs of rainbow trout, and hence in determining social status. Rainbow trout treated with cortisol were significantly more likely to become subordinate in paired encounters with smaller untreated conspecifics.