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The steelhead trout (Oncorhynchus mykiss) is the species most at risk from selenium (Se) exposure in the San Francisco Bay Delta (SFBD). However, although steelhead trout are usually exposed to environmental Se in the juvenile stage, data to test their sensitivity to excess Se, especially its organic form, in the juvenile stage are scarce. Therefore, the objective of the current study was to assess the sensitivity of juvenile steelhead trout to ecologically relevant forms of Se using integrated sensitive endpoints. Fish (mean weight: 22.3 g) were fed one of five diets containing 1.1 (control), 8.8, 15.4, 30.8, and 61.6 μg Se/g diet dw (Se1.1, Se8.8, Se15.4, Se30.8, and Se61.6, respectively) in the form of selenomethionine for 4 weeks. After 4 weeks, Se significantly accumulated in a dose-dependent manner in all tissues at different rates. The growth rate and plasma cholesterol were significantly depressed in fish fed diets containing Se30.8 and above. Hematological parameters and mortality were significantly elevated in fish fed the Se61.6 diet. Marked histopathological alterations were observed in fish fed the Se8.8 diet (the lowest observed effect concentration, LOEC) and above. The current results suggest that the steelhead trout is more sensitive to excess Se than nonanadromous rainbow trout used in previous studies because of its lower LOEC despite the use of selenomethionine and the shorter experimental duration. Additionally, it should be noted that the current Se levels found in the SFBD are already a threat to the threatened population of steelhead trout on the central California coast. • Lowest observed effect concentration of dietary Se in steelhead trout was 8.8 ppm. • The sensitive endpoint was liver histopathologyr. • The selenium levels in the liver were 4–5 times higher than those in the kidney. • Steelhead trout seems more sensitive to excess Se than non-anadromous rainbow trout. • The Se levels in San Francisco estuary is a risk to juvenile steelhead trout. [ABSTRACT FROM AUTHOR]