Nemaline myopathy is among the most common congenital myopathies. We describe for the first time a novel double de novo mutation in two adjacent codons resulting in two amino acid changes E74D and H75Y in the ACTA1 gene. The hypotonic male infant was the first son of healthy unrelated parents with no family history of neuromuscular disorders. Pregnancy was complicated: decreased fetal movements were noted on the 25th week of gestation, premature labour pains were present from the 29th week onwards and because of breech presentation a Caesarian section was carried out in the 39th week. The patient presented with multiple congenital fractures and joint contractures. He was dependent on ventilatory support until his death at 2 months. Muscle biopsy revealed severely atrophic and rounded muscle fibers with considerable variation in diameter and pronounced disorganization of the myofibers. Electron microscopy indicated a distinct disturbance of the myofibrillar architecture and nemaline rods. In view of previously described cases carrying different single missense mutations of the amino acid residues E74 or H75, we suggest that the particular genotype E74D/H75Y is compatible with the severity of the patient's phenotype. The possibility of germ cell mosaicism should be taken into account in genetic counseling.