Nemaline myopathies are diseases characterized by the presence in muscle fibres of pathognomonic rod bodies. These are composed largely of α–actinin and actin. We have identified a missense mutation in the α–tropomyosin gene, TPM3, which segregates completely with the disease in a family whose autosomal dominant nemaline myopathy we had previously localized to chromosome 1p13–q25. The mutation substitutes an arginine residue for a highly conserved methionine in a putative actin–binding site near the N terminus of the α–tropomyosin. The mutation may strengthen tropomyosin – actin binding, leading to rod body formation, by adding a further basic residue to the postulated actin–binding motif.