The non-differentially stained karyotypes of the tropical voles Microtus quasiater (Coues, 1874) and M. umbrosus (Merriam, 1898) are described by first time, and compared and contrasted with that of the hypothetical primitive karyotype for Arvicolinae. The karyotype of M. quasiater from Veracruz, Mexico, has 2n = 62 and FN = 66, while that for M. umbrosus from Oaxaca, Mexico, has 2n = 56 and FN = 60. Their chromosomal morphology comprises few two-armed chromosomes and a relatively large number of single-armed chromosomes. Both species share both a medium-sized metacentric X chromosome and a small telocentric Y chromosome. This karyotypical pattern deviates from the hypothetical primitive condition. However, the karyotypes of other Microtus, including Mexican species of temperate affinity, seem to be even more divergent from the ancestral karyotypical pattern.
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