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A 6-wk experiment was conducted using 360 one-day-old Jumbo Cornish Cross broiler chicks to evaluate the effects of the source and concentration of Se on growth performance and Se retention. Broilers were fed corn-soy-based diets formulated to contain 0 (negative control), 0.1, 0.2, or 0.3 ppm of supplemental Se from SelenoSource AF (Se yeast A, Diamond V Mills, Cedar Rapids, IA), 0.3 ppm of Se from Sel-Plex (Se yeast B, Alltech, Nicholasville, KY), or 0.3 ppm of Se from sodium selenite. Starter diets were fed for the first 21 d (6 replicates of 10 broilers per treatment). On d 21, broilers were regrouped within dietary treatments to finisher cages (11 replicates of 5 broilers per treatment) and fed finisher diets for the following 21 d. Feed intake was measured daily. Body weights of the broilers were measured on d 1, 21, and 42. Excreta samples were collected for 4 consecutive days at the end of each 3-wk feeding period to estimate Se retention. Blood samples were collected from 11 broilers per treatment on d 42 to determine blood Se concentrations and glutathione peroxidase (GPX) activity. Selenium supplementation did not influence (P > 0.05) the growth performance of broilers at 42 d of age. Blood Se and GPX activities increased (P < 0.05) as the concentration of Se in diets increased. Selenium retention as a percentage of Se intake decreased linearly (P = 0.01) as the concentration of Se increased in the diets. At 0.3 ppm, blood Se and GPX activities were higher (P = 0.01) for broilers supplemented with Se yeast A compared with Se yeast B when expressed as relative to Se intake. Selenium retention was greater when organic Se was supplemented (P = 0.01), compared with inorganic Se. Results of the study suggest that Se retention had an inverse relationship with the concentration of Se supplemented and was influenced by the dietary source of Se. Whole blood Se relative to Se intake suggests a higher bioavailability of Se from the organic source compared with inorganic sodium selenite and that differences in bioavailability may exist between organic sources of Se.