Polyploidy, the presence of more than two chromosome sets, is common in plants, but extremely rare in animals. The absence of polyploid organisms with well-differentiated sex chromosomes suggests that the disruption of the dosage between autosomes and sex chromosomes is incompatible with normal development. Thus, the announcement in 1999 of tetraploidy in a mammal, the South American red vizcacha rat Tympanoctomys barrerae, provoked great interest, even though the definitive proof of tetraploidy, the presence of four copies of each chromosome, was never provided. Here we used classical and molecular cytogenetics to test the ploidy level of T. barrerae and demonstrate that only two copies of each chromosome are present in this karyotype. The red vizcacha rat is clearly diploid and the amplification and dispersion of repetitive sequences best explain the large genome size of this mammal. Thus, polyploidy in mammals remains as unlikely as it has always been.