Characteristics of spermatogonia were determined in the C57BL/6J strain mouse using high-resolution light microscopy of plastic-embedded tissues and identifying cells during stages of the spermatogenic cycle. The frequency of expecting each spermatogonial cell type was a major factor in identifying and categorizing various cell types. Although numerous characteristics were described, several major differences were noted in spermatogonial cell types. The group comprising A_s, A_pr, and A_al spermatogonia could be differentiated based primarily on mottling of heterochromatin throughout the nucleus in the absence of heterochromatin lining the nuclear envelope. The A₁ cells displayed finely granular chromatin throughout the nucleus and virtually no flakes of heterochromatin along the nuclear membrane. The A₂ through A₄ spermatogonia contained progressively more heterochromatin rimming the nucleus. Intermediate-type spermatogonia displayed flaky or shallow heterochromatin that completely rimmed the nucleus. Type B spermatogonia showed rounded heterochromatin periodically along the nuclear envelope. Use of gray-scale histograms allowed objective quantification of nuclear characteristics and showed a logical shift in the gray scale to a narrower and darker profile, from four cell types leading to A₁ cells. The ability to differentiate spermatogonial types is a prerequisite to studying the behavior and kinetics of the earliest of the germ cell types in both normal and abnormal spermatogenesis.