Crystalline inclusions were found in the muscle of a previously healthy 63 year old male with atypical postinfectious polyradiculoneuropathy. Approximately 12 days after the onset of the illness, biopsy of biceps femoris and vastus lateralis muscles revealed, in addition to changes consistent with early neurogenic atrophy, inclusions measuring 22 micronm in greatest dimensions. In plastic-embedded toluidine blue stained sections the translucent ovoid to angular inclusions were seen in a subsarcolemmal position, adjacent to nuclei. Ultrastructural examination revealed a crystalline structure consisting of parallel dense lines measuring approximately 70 A in diameter with an axial periodicity of 130 A. Unlike rods, the crystalline inclusions were not continuous with Z-discs; rather they displaced contractile and other cellular elements. Their appearance was similar to intramuscular crystals in diabetic amyotrophy and to vincristine-induced and naturally occurring intracytoplasmic crystalline structures. The nature of the crystalline inclusions and their relationship to the basic disease process was not established.