The size of the metathoracic dorsal longitudinal flight muscle (DLM) in the cricket Teleogryllus oceanicus remains a constant proportion of metathoracic volume during most of nymphal life. Between the middle of the last nymphal instar and the fifth day of adult life, the DLM grows more rapidly than the metathorax, so that the ratio of DLM weight to metathoracic volume increases by a factor of 4. During this time, the DLM changes in appearance from translucent to pink, indicating an increase in mitochondrial content. Isometric twitch duration remains unchanged through late nymphal and early adult life (21 msec, onset to 50% relaxation, 25°C), as does fusion frequency (approximately 100 Hz, 25°C). During late adult life, DLM size decreases to 13% of the average for a young adult, isometric twitch duration doubles, and the DLM changes in appearance from pink to white. Since the flight motor program develops late in nymphal life and wings are produced only at the terminal molt, contraction kinetics that are characteristic of adult flight muscles appear to develop before the DLM receives the neural activity pattern for or becomes functional in flight. The appearance of adult contraction kinetics is apparently controlled by factors different from those that control increase in muscle size and mitochondrial content, since these processes are temporally separated during development.