The effects of dietary supplementation of selenium in an organic form on performance, carcass traits and selenium contents in tissues of broiler cockerels randomly divided into 3 groups: group I. (control), without Se supplement; experimental groups: II.? 0.3 mg Se/kg (Se-enriched yeast), III. ? 0.3 mg Se/kg (Se-enriched alga Chlorella), were studied. Significantly higher live weights of broiler chickens were recorded in the experimental groups (II ? 2 430.6 g and III ? 2 425.2 g). There were no significant differences between the groups in feed conversion, carcass traits, dressing percentage and mortality. The contents of selenium in breast and thigh muscle, liver, feathers and excrements in both experimental groups compared to the control group werehigher. By comparison of both experimental groups the higher