At 110 d of gestation, fetuses were removed from sows selected for high (obese) or for low (lean) backfat thickness. The body weights of lean (1,031 +/- 64 g) and obese (864 +/- 55 g) fetuses were not significantly different. Analysis of muscle composition and of in vitro metabolic characteristics was conducted on the biceps femoris muscle. The percentage of dry weight, protein and glycogen was greater in the muscle of obese fetuses than in the muscle of lean fetuses (P less than .01, P less than .05, and P less than .05, respectively). Percentage of muscle triglyceride was similar (P greater than .05) between the two phenotypes. The rate of glucose oxidation to CO2 tended to be greater (P less than .07) and the rate of lactate production was lower (P less than .05) in the muscle from obese fetuses than in the muscle from lean fetuses. The rates of leucine oxidation to CO2 and of palmitate oxidation to CO2 did not differ between phenotypes. The rate of alpha-ketoisocaproate release from the muscle of obese fetuses was greater (P less than .05) than from that of the lean fetuses. The rate of release of alanine and of glutamine plus glutamate did not differ between phenotypes. The rate of esterification of palmitate did not differ between phenotypes. It was concluded that abnormalities in glucose metabolism and in the partitioning of leucine between oxidation and release as the keto acid existed at 110 d of gestation in the muscle of obese fetuses. Any relation between these differences and ultimate differences in carcass composition were not evident.