Sperm quantity and quality were examined in 2 populations (from Mar de Cobo and Necochea, Argentina) of the subterranean rodent Ctenomys talarum (tuco-tuco) for which polygyny was confirmed as their mating system. Nonetheless, differential population attributes allow us to propose that polygyny may be exerted at different levels. Mean epididymal sperm counts were not different, but the Necochea population was characterized by higher variance in sperm counts. Low sperm production, with little variance, in the Mar de Cobo population is consistent with a more extreme polygyny, with aggression playing an important role in the males securing monopolization of females. In the Necochea population, where males were less likely to maintain female exclusivity, subordinant males are not driven away and may invest more in sperm production than in male–male interactions. However, C. talarum males from both populations produce high-quality sperm cells, based on their motility, percentage of live sperm cells, and morphology.