The fibrotic lateral rectus muscle and the medial rectus muscle of 4 patients with esotropia and myopia who underwent surgical correction were studied ultrastructurally. All patients revealed underactivity of the lateral rectus muscle. In 3 patients the underactive lateral rectus was completely fibrotic; in 1 patient there was 80% fibrosis, and the rest of the muscle showed different stages of degeneration. The fibrotic lateral recti were composed of collagen fibrils and remnants of degenerated organelles. The collagen fibrils of the lateral recti were of different sizes, and the mean diameter varied from 65 to 92 nm. On the other hand, the interstitial collagen fibrils of nonfibrotic medial rectus muscles of these patients and also of control muscles showed less variation in size, and the mean diameter was smaller as compared with the fibrotic lateral rectus. It is suggested that the collagen fibrils that compose the fibrotic lateral rectus muscle probably develop under conditions which differ from the normal development of collagen fibrils.