The adult male viscacha (Lagostomus maximus maximus) is a seasonal rodent. It exhibits a short period of testicular regression with partial arrest of spermatogenesis during winter (July-August). The present study provides the first description of the viscacha spermatogenic cycle during the period of maximum gonadal activity (summer-autumn). The testes were processed by using conventional techniques of light and electron microscopy. One-micrometer-thick sections stained with toluidine blue were used to clearly define the different cell associations. Spermatogenesis in this rodent is well organized and synchronized and has been divided into nine stages. The present classification is based on the morphogenesis of the acrosomal system (stages I-V) and the degree of elongation and condensation of the heads of the differentiating spermatids (stages VI-IX). Stage I is principally defined by round spermatids with a well-developed, juxtanuclear Golgi complex and without acrosomal components that are visible under the light microscope. Sperm release from the viscacha seminiferous epithelium occurs either in late stage III or in early stage IV. Stage IX is characterized by diplotene spermatocytes, figures of meiosis I or II, and secondary spermatocytes.