• Ninety extraocular muscle (EOM) biopsy specimens from 80 children with strabismus and normal ocular alignment were studied by light microscopy, enzyme histochemistry, and electron microscopy. Light microscopy showed considerable variation in muscle fiber shape and size with sarcomere disruption, sharp increases in endomysial and perimysial collagen, numerous vacuoles and subsarcolemmal inclusions. Electron microscopy showed disruption of myofilaments, nemaline rods, abnormal mitochondria, leptomeric profiles, occasional "myelin figures," glycogen, and lipid-like droplets. Some intramuscular nerves contained long-spacing collagen ("Luse bodies"). Unlike somatic skeletal muscle, EOM showed a consistent lack of mosaic pattern and reciprocal stain activity between fiber types noted by enzyme histochemistry. Quantitative observations indicate that minimal but consistent morphologic differences exist between strabismic and nonstrabismic EOM. The role these changes play in the pathogenesis of strabismus and their significance remains unclear.