The most common repeated nucleotide sequences of the highly repetitive satellite HS-α fraction from kangaroo rat Dipodomys ordii was determined using ribosubstitution methods. This sequence was α nucleotides long and represented about 25% of the total HS-α satellite DNA, while the remaining DNA was composed of sequence variants related to the most common sequence. The sequences of the commonest of these variants are reported. Furthermore, the most common repeated sequence was identical to that reported for the α satellite of guinea pig Cavia porcellus. The α satellites of guinea pig, Cavia porcellus, pocket gopher, Thomomys bottae and antelope ground squirrel, Ammospermophilus leucurus, are shown to have sequences in common with the kangaroo rat. This implies that the simplest repeated sequences of mammalian satellite DNAs may persist over much longer evolutionary times than previously thought. Attempts to explain the very rapid quantitative changes in satellites whose sequence is strongly conserved have led us to consider that they might have a role in sympatric speciation. Among the novel features of the model presented is that fluctuations in satellites could be due to “speciation genes.” Such genes would confer a strong selective advantage in certain situations, and could explain the many puzzling instances in which large numbers of new related species have appeared over a short evolutionary span.