Male sheath-tail bats were collected from central Queensland over a 12-month period. Plasma testosterone levels peaked in August, coincident with an increase in the volume of the accessory glands and ampulla/seminal vesicle secretion. Peak spermatogenesis occurred in summer and autumn and declined in the face of maximal testosterone levels in winter. Levels of androstenedione and 5 alpha-dihydrotestosterone were high compared with testosterone levels and showed no significant seasonal changes. Ultrastructural examination of Leydig cell cytoplasm revealed numerous lipid droplets and mitochondria, and an abundant smooth endoplasmic reticulum. There were no seasonal changes in Leydig cell ultrastructure. The anomalous reproductive pattern in this species is consistent with the imposition of a cold-induced winter spermatogenic shutdown, on a framework of continuous spermatogenesis, with spring peaks in testosterone and accessory gland activity.