Histochemical changes in the mother of a patient with nemaline myopathy were used to identify her as the gene carrier even though rod-bodies were not present in her muscle biopsy and she was not weak. The patient and her mother both had marked type I fiber predominance with large groups of type I fibers present. Histochemical changes known to occur in nemaline myopathy include smallness and predominance of type I fibers. Such changes support the concept that this disease may result from subtle defects in innervation since fiber types are determined by innervation. Although this disease is thought to be transmitted by autosomal dominant mode, lack of male-to-male transmission and a predominance of female cases in the literature suggest that this may be (1) an X-linked dominant, (2) a sex-influenced autosomal dominant, or (3) an autosomal dominant which is semilethal in males. The family described here is the first in which a presumably affected parent showed only the histochemical change without rod-bodies, thus emphasizing the importance of histochemical evaluation of relatives' biopsies for genetic counseling.