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We have partially characterized four Drosophila melanogaster alpha-actinin gene mutants, I(1)2Cb1, I(1)2Cb2, I(1)2Cb4, and I(1)2Cb5. We demonstrate that in each case the mutation is caused by a chromosomal rearrangement that precludes normal protein synthesis. In the absence of alpha-actinin, flies complete embryogenesis and develop into flaccid larvae that die within approximately 24 hr. These larvae have noticeable muscle dysfunction at hatching, although they, nevertheless, are capable of escaping from the egg membranes and of subsequent crawling movements. During larval development muscles degenerate, progressively limiting mobility and ultimately causing death. Electron microscopy of mutant muscle fibers reveals that myofibrils are grossly disrupted in one day old larvae and that electron-dense structures reminiscent of those seen in human nemaline myopathies are present throughout larval life. Our work rigorously demonstrates that alpha-actinin deficiencies are the cause of I(1)2Cb muscle defects. We anticipate that the alpha-actinin mutants described herein will facilitate in vivo tests of spectrin superfamily protein domain functions using a combination of directed mutagenesis and germline transformation.