The interactions of sublethal waterborne copper exposure and social dominance behavior were examined in juvenile rainbow trout. Dominance hierarchies were determined between pairs of fish by behavioral observations and among groups of 10 fish by the use of passive integrated transponder (PIT) tagging equipment. The present study is one of the first to utilize this novel PIT tag method for behavioral assessment. Feeding behavior was quantified by placing a PIT tag recorder at the entrance to the feeding area. Linear dominance hierarchies were documented based on these observations of feeding behavior. Dominance hierarchies established in control water were not altered by exposure to 30 microg/L of copper; however, physiological responses of fish to sublethal concentrations of copper were related to social rank. Subordinate fish exhibited a higher accumulation of copper in both gill and liver tissue. Subordinates of paired fish were also shown to have a higher uptake of sodium than dominant fish, and the uptake of sodium was correlated with uptake of copper from the water. Therefore, within a population of fish, it cannot be assumed that individuals of different social status will exhibit the same physiological responses to the presence of copper.