A three-year experiment was conducted to compare the performance of Suffolk- and Southdown-sired lambs in an area characterized by a relatively short period of good feed supply. One hundred whiteface ewes of mixed breeding were allotted each year to three or to four rams of each breed. The three-year average results, for Suffolk- and Southdown-sired groups respectively, were as follows: lambs weaned per ewe alive at lambing, 0.85 and 0.91; 120-day lamb weight, 71.3 and 59.3 lb.; percentage “fats” at weaning, 24 and 30; daily gain on irrigated pasture, 0.31 and 0.23 lb.; dressing percent (based on unshrunk live weights), 48.3 and 49.1; percent of carcasses grading prime, choice, good and utility, 0, 59, 38, 3 and 7, 73, 18 and 2; carcass price per pound (gross), $0.40 and $0.41; carcass price per pound (net after killing charges deducted), $0.36 and $0.36; returns per ewe alive at lambing, $13.15 and $11.38. The results indicate that the lighter weights of the Southdown-sired lambs were not compensated for by their slightly higher carcass grades and percentage survival to weaning, and that the lambs sired by rams of the larger breed returned approximately 16% more lamb income per ewe.