A primary goal of animal agriculture is to improve the efficiency of meat production by optimizing the growth performance of livestock. Divergent genetic selection of swine for improved residual feed intake (RFI) has resulted in a line of pigs selected for low RFI that reaches the same market weight while consuming 10 to 15% less feed than the high RFI line. Our long-term goal is to define the contribution of mitochondria function to this improved growth efficiency. Investigations using RFI selection have demonstrated that muscle mitochondria from pigs in the low RFI line exhibited less electron leakage from the electron transport chain. This is particularly evident in the mitochondria from the red portion of the semitendinosus (complex I and II) and the white portion of the semitendinosus (complex I, II, and III). Reactive oxygen species production from electron leakage in the mitochondria was positively correlated with RFI, demonstrating a link between poorer mitochondria function and poorer growth efficiency. A parallel observation was that muscle from more efficient pigs had a greater capacity to decrease protein degradation and conserve muscle mass. Mitochondria from muscle of the more efficient low RFI pigs had a greater abundance of heat shock protein 70, heat shock protein 60, malate dehydrogenase, ERO1 α, and subunit 1 of the cytochrome bc1 complex. Mitochondria from muscle of the less efficient high RFI pigs had a greater abundance of pyruvate kinase and glyceraldehyde 3-phosphate dehydrogenase. It is important to note that post-translational modification of proteins plays a central role in mitochondria function. Heat shock protein 70, heat shock protein 60, and ATP synthase are among the mitochondria proteins modified by phosphorylation. The combined evidence demonstrates that muscle growth and maintenance is directly influenced by mitochondria efficiency and cellular protein homeostasis. (Funded in part by the AFRI competitive grant number 2010–65206–20670 from USDA NIFA and by Iowa Pork Producers grant number 10–009).