Analyses of qualitative and quantitative variation in bacula and soft parts of the glans penis of 13 species of Ctenomys Blainville, 1926 from Argentina were used to suggest systematic and evolutionary relationships. The 13 species can be divided into spike-bearing {C. australis, C. azarae, C. porteousi, C. rionegrensis, and C. talarum) and spiny bulb-bearing species {C. dorbignyi, C. pearsoni, C. perrensi, C. roigi, Ctenomys sp. from Curuzú Laurel, Ctenomys sp. from M. F. Mantilla, and Ctenomys sp. from San Roque). Ctenomys yolandae is unique because it shows both spikes and spiny bulbs. In addition to spikes and spiny bulbs, some populations of C. pearsoni, C. talarum, and C. yolandae had a new structure, an inner sac of the intromittent sac. The most frequent pattern of occurrence of spikes or spiny bulbs was 1-1 (one at each side of the urethra), an ancestral character state in caviomorphs. Variation in bacular dimensions was limited and differences among species were small, with the exception of C. pearsoni. This species showed a significantly short baculum with a wide base. Sperm and penial morphology suggests that C. talarum is the most plesiomorphic and C. yolandae the most derived species of this group. The pattern of geographic variation among these 13 species rejects the hypothesis of penis morphology contributing to reproductive isolation. Ctenomys pearsoni is the only species with some evidence of reproductive isolation resulting from penis morphology.
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