Rainbow trout of two strains (Shasta and Idaho) were fed two diets (Colorado and Santa Monica) under experimental conditions. The effects of diet, strain, sex, and age on total blood cell count, hematocrit, hemoglobin, sedimentation rate, bilirubin, alkaline phosphatase, glutamic‐oxalacetic transaminase, total protein, glucose, cholesterol, creatinine, and uric acid were measured. No sex‐related differences were detected in hematological values. Variations due to time appeared to be random. Variations due to diet, strain, and/or diet‐strain interactions were found for many of the blood components. There were differences in several tests between hematological characteristics of “wild” and cultured trout. Hematological effects of diet, strain, age, water temperature, sampling techniques, and test methods were important, and should be considered by researchers. The range of mean values obtained for the blood components measured can be used as “normal” levels for comparison if investigators employ similar conditions and techniques.