The tuco-tuco das dunas (Ctenomys flamarioni) is a subterranean rodent endemic to sand-dune ecosystems along the southern coast of Brazil. We studied 3 populations that differed in the degree of human impact, and used direct and indirect methods to assess demographic and genetic information. Field studies revealed a tendency toward a female-biased sex ratio, and sexual dimorphism in both weight and length in all 3 populations. This evidence supports a hypothesis of polygyny in C. flamarioni. Using 9 microsatellite loci, we explored patterns of variation and genetic structure among the populations. Our findings suggested that the Xangri-lá and Remanso populations, living in more-disturbed locations, could have experienced demographic reductions in population size, but not the Pinhal population. However, other factors such as a polygynous breeding system and the environmental instability that characterizes the coastal dunes may have influenced the observed pattern. Analysis of genetic structure revealed strong differentiation among populations, but no significant structure at the intrapopulation level. Nonsignificant values for the tested indices from assignment tests (F_ST, F_IS, mAIc, and vAIc) over population showed no evidence of sex-biased dispersal. The same was observed from analyses of molecular variance. Nevertheless, lower pairwise F_ST and higher Nm values between males from Xangri-lá and Remanso indicated greater gene flow among males, suggesting a slightly male-biased dispersal pattern. Significant differences in interpopulation dispersal patterns were clearer among females, with greater dispersal for females from Pinhal than those at Remanso and Xangri-lá.