We describe the use of a green fluorescent protein (GFP) reporter construct to monitor indirect flight muscle development in normal and mutant Drosophila melanogaster strains. We used polymerase chain reaction to amplify a portion of the Act88F actin gene that includes 1420 nucleotides of flanking DNA, the transcription start, first intron, and initiator codon, incorporating the fragment into the Drosophila germ line transformation vector pCaSpeR. We fused the fragment to the gene encoding green fluorescent protein of the bioluminescent jellyfish Aequorea victoria. We could detect GFP protein in transgenic strains and found that its accumulation, conveniently visualized in living flies using epifluorescence microscopy, was limited to the indirect flight muscles. GFP fluorescence can be used to visualize all stages of flight muscle development subsequent to myoblast fusion nad facilitates the detection of morphological changes in fibers caused by particular mutations.