As the maturation of honeybee flight muscle proceeded, distinct patterns in the development of the energetic capabilities of the mitochondria were apparent. Mitochondrial ATPase and ATP-Pi exchange activities were evident in newly emerged bees and exhibited generally parallel increases in activity with increasing age. Respiratory control by ADP concentration, however, was not apparent in mitochondrial preparations from young, 1-day- and 4-day-old bees. Also, phosphorylation linked to the oxidation of succinate, α-glycero-P or pyruvate + malate was either not apparent or at best negligible during this period of adult life. This, however, may be due to an age-related fragility of the mitochondria rather than to an actual lack of these attributes. The mitochondrial α-glycero-P oxidase system appeared to exhibit considerable activity even during the early stages of adult life after emergence. Most likely, the operation of this system is responsible for mediating a significant portion of the energy production for the limited flight activity exhibited by bees during the early stages of adult life. The pyruvate metabolizing system was not fully functional until relatively late in adult maturation (16–20 days after emergence). This would appear to indicate that sustained flight, exhibited by fully matured flying bees, is possible only when the Krebs cycle is in a fully functional state.