Adult male and female cockroaches, Periplaneta americana, differ in their capacity for sustained flight. Tethered 20 day adult males flap their wings about 15 min; females 12 seconds. Males are less massive and have greater total wing area than do females, with wing loading only about half that of the female. The basalar muscle, a major wing depressor in flight, is also sexually dimorphic. In males it is 1.5× more massive and has a cross sectional area 1.2× greater than the female homologue. Muscle fibers of the adult male and female basalar muscles are ultrastructurally and biochemically distinct. Female fibers have a myofilament array similar to that of other non‐flight locomotory muscles, with 10–12 actin filaments surrounding each myosin filament. Adult male fibers typically have six actin filaments around each myosin filament. The male fibers also appear to be more oxidative. The volume density of mitochondria is about 3× greater and the specific activity for citrate synthase, an oxidative indicator, is 3.5× greater in male fibers than in female fibers. Male basalar muscle shows an enhanced ability to resist fatigue when its motoneurone is stimulated at flight frequency; homologues from the male terminal nymph and 20 day adult female are both highly fatiguable. The basalar muscle of both male and female terminal nymphs are similar in ultrastructural and biochemical profile and are not greatly different from the adult female. In male roaches there may be important neuronal or hormonal inputs at about the time of the terminal molt to adulthood that direct the transformation of the basalar muscle in time for the assumption of a new behavioral role—flight. © 1994 Wiley‐Liss, Inc.