The organization of the retinal nerve fiber layer of the cynomolgus monkey was studied by electron microscopy. Nerve fiber size spectra were obtained from measurements of every fiber in enlarged montages of selected bundles. Variation in spectra among nasal, arcuate, and papillomacular bundles was examined, and variation of spectra of a given bundle as it crossed the retina was determined. Among the three types of bundles, the papillomacular contained relatively more small fibers, nasal bundles relatively more large fibers. Systematic variation of fiber spectra was observed as the disc was approached by arcuate and papillomacular but not nasal bundles. Arcuate bundles sampled in the peripapillary area contained relatively more small fibers than when sampled at a greater distance from the disc. In contrast, papillomacular bundles sampled near the disc contained relatively more large fibers than near the fovea.