SUMMARY Malignant mesenchymoma within the right ulnar nerve of an 8 year old boy is described. The patient did not have stigmata of von Recklinghausen's neurofibromatosis. The growing and painful tumour was excised five and a half months after detection, and recurred five months later. Mingling of the nerve sheath sarcoma and rhabdomyosarcoma was noted within the same mass which was separated from the adjacent striated muscles. It is suggested that this mesenchymoma arose from mesenchymal cells or cells of mesenchymal type comprising the peripheral nerve sheath which is derived from ectomesenchyme of the neural crest.We have already recorded two cases of primary intracranial rhabdomyosarcoma (Shuangshoti et al, 1968;Shuangshoti and Phonprasert, 1976). This tumour comprised only 0.1 % of 1028 intracranial neoplasms reported by Shuangshoti and Panyathanya (1974). These authors have also investigated 1274 tumours of peripheral nerve-61.5 % neurofibromas, 33.5 % neurilemmomas, 0.1 % combined neurofibromas and neurilemmomas, and 4.9 % nerve sheath sarcomas (so-called malignant Schwannomas, neurofibrosarcomas, or neurogenic sarcomas). They found no instances of these peripheral nerve neoplasms containing elements of striated muscle. Only a subcutaneous neurilemmoma had bone formation within the mass which was presented separately