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Hormonal control of flight muscle development was studied in the desert locust, Schistocerca gregaria through quantitative analysis of electron micrographs from normal adults, precocious adults (obtained after ethoxy-precocene treatment), as well as supernumerary nymphs (obtained by methoprene treatment) and azadirachtin-induced overaged nymphs. During the first 2 weeks after imaginal ecdysis, flight muscle of normal adults as well as its constituent mitochondria and myofibrils increased in size. The mitochondrial portion of the tissue enlarged from 17 to 30%. Similar growth of all components was observed in precocious adults, obtained after treatment of 3rd-instar nymphs with ethoxy-precocene. Overaged nymphs as well as 6th-instar, supernumerary nymphs did not exhibit growth of myofibrillar components; size and distribution of mitochondria however was similar to adult insects. The results indicate that metamorphosis is required for complete muscle differentiation and development. Mitochondrial growth, however, is largely independent of endocrine influences. The results are discussed in light of published biochemical findings.