Electron microscopic study of livers from mice fed 167 ppm polybrominated biphenyl (PBB) revealed mitochondrial abnormalities which consisted of both alterations in size and the formation of crystalline-like inclusions within the mitochondrial matrix. These inclusions appeared as parallel arrays of rods and were found in elongated mitochondria which contained few cristae. The findings are briefly described and the possible significance of such inclusions in relation to mitochondrial aberrations are discussed.