Studies on Thomomys and Peromyscus, among others, have confirmed that karyotypic variation in mammals is often reflected in genome size variation. We used flow cytometry to estimate genome sizes from 91 individuals of the karyotypically variable Graomys griseoflavus complex from Argentina. Our samples represented 16 populations and included 2 ostensibly reproductively isolated forms. The mean genome size (2C value) for all populations, 5.72 pg ± 0.34 SD of DNA, was low for mammals, and the range was greater than observed in previous studies on Geomys, Thomomys, and bats. Two groups of populations were identified that differed in mean genome size. A high–genome-size group consisted of 2 populations in San Juan Province with a mean genome size of 7.38 ± 0.53 pg. These populations occupied the most arid part of the Monte Desert. A small–genome-size group included the remaining 14 populations that mostly occupied a broadly distributed, continuous habitat and possessed lower genome sizes with a mean of 5.62 ± 0.56 pg. Population means in this group ranged from 2C = 4.97 to 6.51. Previous studies have shown the existence of 2 reproductively isolated and karyotypically differentiated species within the G. griseoflavus complex. The high–genome-size group seems to correspond to the species with 2n = 42 (G. centralis) and the low–genome-size group to the species with 2n = 36–38 (G. griseoflavus). Flow cytometry is a potentially useful method to rapidly assess differentiation among populations of the G. griseoflavus complex.