The presence of the organochlorine pesticide endosulfan in the water column confers a significant direct risk to the biota therein, yet relatively little is known regarding the toxic impact of dietborne endosulfan to aquatic organisms. Pre-smolt Atlantic salmon (Salmo salar) were fed at levels of technical endosulfan equal to the European Union regulatory limit (5 microg kg(-1)) and at levels 10 and 100 times greater, for a total of 49 days with haematology (e.g. erythrocyte count, haemoglobin, haematocrit, white blood cell composition), blood biochemistry (e.g. serum aminotransferase, plasma ions) and growth parameters (e.g. condition factor) recorded at days 0, 14, 35, and 49. Toxicological assessment of the individual alpha and beta-isomers that comprise technical endosulfan was also attempted. No mortality was observed in any group during the experiment. In the groups exposed to the control, 5, and 50 microg kg(-1) technical endosulfan feeds no significant alterations in any measured parameters were determined at any time point. No differences were observed between the technical mixture and the individual alpha and beta-isomer treatment groups. Condition factor was significantly reduced in fish exposed to 500 microg kg(-1) at day 49, while haematocrit, haemoglobin and mean corpuscular haemoglobin were significantly elevated after 35 days of exposure to the same dietary level, but returned to control levels by day 49. The present study shows that with regards to acute toxicity, Atlantic salmon are able to tolerate dietary technical endosulfan levels up to 500 microg kg(-1).