Eight X-chromosome mutations (falling into five complementation groups) that affect the development and morphology of the indirect flight muscles of Drosophila melanogaster were investigated using histological, behavioural and genetic techniques. All of these mutations result in flightlessness, in a marked reduction in the ability of the flies to jump, and in the wings being held in abnormal positions. Mutations in each of the complementation groups have different effects on the morphology of the muscles. Two (flapwing, vertical wing) result in absence of most of the indirect flight muscle fibres, a third (upheld) is required for the gross organization of muscle structure, another (heldup) is involved in the maintenance of muscle structure once formed, and the fifth seems to be necessary for the detailed architecture of the muscle fibre (indented thorax). The analysis of flies genetically mosaic with respect to each mutation by the technique of fate-mapping suggests that three (heldup, upheld and indented thorax) of the genes concerned have their primary site of action in the musculature itself, while the other two (flapwing and vertical wing) may function primarily in the fat-body and tracheae respectively.