The objective of the present study was to estimate the allele and genotype frequencies of the CSN3/HinfI and LGB/HaeIII gene polymorphisms in beef cattle belonging to different genetic groups, and to determine the effects of these polymorphisms on growth and carcass traits in these animals, which are submitted to an intensive production model. Genotyping was performed on 79 Nelore, 30 Canchim (5/8 Charolais + 3/8 Zebu) and 275 crossbred cattle originating from the crosses of Simmental (n = 30) and Angus (n = 245) sires with Nelore females. Body weight, weight gain, dressing percentage, longissimus dorsi area and backfat thickness were fitted using the GLM procedure, and least square means of the genotypes were compared by the F test. The results showed that the CSN3/HinfI and LGB/HaeIII polymorphisms did not have any effect on growth or carcass traits (p > 0.05).Key words: beef cattle, polymorphisms, candidate gene, growth, carcass. "Super-young" steers are animals finished on feedlot immediately after weaning and slaughtered as 13 montholds at the most, weighing at least 450 kg. These animals should also have backfat thickness above 3.0 mm, which, associated with the animals' youth, will guarantee the quality of meat and its by-products. This system takes advantage of maximum biological efficiency, which can basically be defined as live weight gain over energy intake. It provides an excellent model of intensive cattle rearing.The κ-casein (CSN3) and β-lactoglobulin (LGB) genes are expressed in milk and are important in the evaluation of the milk production potential and milk fat and protein percentage, indicating a maternal ability in beef cattle and therefore representing candidate genes for growth traits. In addition, these genes can also be associated with other loci that have a direct influence on growth. Polymorphisms of these genes have been associated with milk production and quality and weight gain until weaning in dairy cattle. However, the results of these studies are contradictory. On the other hand, the effects of milk protein polymorphisms on production traits in beef cattle have been little investigated. Moody et al. (1996) observed an effect of a κ-casein gene polymorphism on growth traits in Hereford beef cattle.The objectives of the present study were to estimate the allele and genotype frequencies of CSN3 and LGB gene polymorphisms in beef cattle belonging to different genetic groups and to determine the effects of these polymorphisms on production traits in these animals which are submitted to a "super-young" production model.For this purpose, blood was collected from 384 bulls belonging to four different genetic groups and differing in frame size, including 79 Nelore, 30 Canchim (5/8 Charolais + 3/8 Zebu) and 275 crossbred cattle originating from crosses of Simmental (n = 30) and Angus (n = 245) sires with Nelore females.Calves were weaned at 210 days of age using a creep-feeding system. At the beginning of the study, the animals were individually identified, treated against endoand ...