This study investigated the dynamics of 2,4-dichlorophenoxyacetic acid (2,4-D) in Salmonid fish. Similar to mammals and marine fish, trout excrete 2,4-D via the urine, with an elimination half-life of 2.4 hr. Although the amount of 2,4-D found in bile was always less than 1% of the dose, the concentration of 2,4-D in bile was greater than any other tissue or fluid four hr or more after exposure. From 20 to 96 hr following administration, bile was the only tissue or fluid which contained detectable amounts of 2,4-D. The results demonstrate that rainbow trout eliminate 2,4-D very rapidly via urinary excretion. However, detectable levels of 2,4-D may be found in the bile for many hours after it is eliminated from all other tissues.