Tuco-tucos, South American rodents of the genus Ctenomys, are an interesting model of speciation. Their strict territorial and solitary underground life, vast but highly fragmented habitats, and low migration activity were the causes of their very fast radiation. About 60 species of this genus have been described. They are found in a variety of habitats, from the mountains of the Andes to the coastal dunes of the Atlantic and from the humid steppes of Pampas to the dry deserts of the Chaco. Tuco-tucos have a very high level of chromosomal polymorphism and polytypism based on Robertsonian and whole-arm reciprocal translocations and inversions. Therefore, they can be used to test different versions of the chromosomal speciation hypothesis. The classic version of this hypothesis emphasizes the sterility of hybrids due to an incorrect or incomplete chromosome synapsis in the chromosomal heterozygotes for rearrangements, germ cell death, chromosome nondisjunction, and the generation of unbalanced gametes. The modern version of the chromosomal speciation hypothesis suggests that the reduction of gene flow across chromosomal hybrid zones is due to the suppression of recombination in hybrids around the breakpoints of the rearrangements distinguishing the parental species. Field studies have revealed no strong negative effects of chromosomal heterozygosity on the fitness of the carriers. These results cast doubt on the validity of the classic version. Analysis of the chromosome behavior in meiotic prophase in chromosomal heterozygotes has revealed significant changes in the frequency and distribution of recombination: crossover suppression around breakpoints and chiasma distalization. These changes can modulate the gene flow between chromosomal races and amplify the divergence that has arisen due to spatial isolation. These data corroborate the recombinational model of chromosomal speciation.