Satellite DNAs are large fractions of the eukaryote genome, made up of short nucleotide sequences very highly repeated in a more or less identical manner. They are located in the constitutive heterochromatin mainly in the pericentrometic regions of the chromosomes, and are species specific. Related species might have similar satellite DNAs. Their main function appears to be the pairing of homologous chromosomes in meiosis. This function is probably related to the repetitiveness more than to the nucleotide sequence of satellite DNAs. Genetic mutations might be not strictly selected by natural selection along satellite DNAs which on the basis of the simplicity of their sequence are genetically inert, do not code for any protein and are not even transcribed in vivo. Satellite DNAs can therefore accumulate mutations which are then multiplied many times by saltatory replication or other mechanisms. This could explain the great differences in base composition of satellite sequences in species even strictly related phenotipically. Here it is proposed that satellite DNAs function as sterility barriers between diverging incipient species. Since many other mechanisms of reproductive isolation (mainly behavioral, ethological, mechanical and environmental) have been developed, species may diverge into two different species without sterility barriers. In these cases satellite DNAs might be still very similar in closely related species. Satellite DNAs functioning as sterility barriers might therefore be a relatively old mechanism of reproductive isolation. As such they seem not to be adaptive, and to be a means of speciation independent of phylogenetic evolution. They might have appeared in evolution with sexual reproduction, to which their appearance might be related in an unknown manner perhaps through subsequent unequal crossing-over, a mechanism of multiplication which might be an alternative to saltatory replication.