There is considerable concern that endocrine disrupting substances such as 4-nonylphenol (4-NP) in the freshwater environment may have adverse effects on the growth, survival, and osmoregulatory ability of salmonids during and after their transfer to sea water. This study was conducted to examine the effects of dietary exposure of coho salmon (Oncorhynchus kisutch) to 4-NP during the parr-smolt transformation phase of their life cycle. Under laboratory conditions, juvenile fish were fed by hand twice daily to satiation diets dosed with one of several concentrations of 4-NP (doses varied between 0 (control) and 2000 mg/kg) for 4 weeks, then immediately transferred to sea water. Growth was observed for two successive 6-week periods following sea water transfer when all groups were fed the control diet (no supplemental 4-NP) only. In addition to 4-NP measurement in fish tissues, thyroid hormone concentrations in blood plasma were followed and related to diet treatment and sampling time. Dietary treatment of 4-NP did not influence the growth and smoltification of coho salmon, a result that conflicts to some extent with other reports in which deleterious effects of water-borne 4-NP on the smoltification process of salmonids were linked to disruption of the endocrine system. Appreciable concentrations of 4-NP were present in the livers, gall bladders and tissues after the 4-week exposure of coho salmon to the highest dietary dose of 4-NP, but 4-NP appeared to be effectively eliminated from the fish by the biliary-fecal pathway after sea water transfer.