The phylogenetic relationships of three plecotine bat genera were studied by comparison of chromosome banding patterns. The G-banded karyotypes of Euderma maculatum and Idionycteris phyllotis were found to be very similar, differing by only two pericentric inversions. Plecotus townsendii possessed a strikingly different G-band karyotype, which differed from that of Euderma and Idionycteris by several fusions and an inversion in the X chromosome. Only five pairs of biarmed chromosomes and two pairs of small acrocentric chromosomes were shared as direct homologs between Plecotus and the other two genera. These results indicate a more complex derivation for the Plecotus karyotype than indicated by Williams et al. (1970) or Bickham (1979). The degree of non-matching of biarmed chromosomes indicates that the ancestor that gave rise to the Plecotus line and the Eudermaj Idionycteris line possessed a karyotype with at least 15 pairs of telocentric or acrocentric chromosomes and a diploid number of at least 42. This ancestral line may have been characterized by a Myotis-like karyotype. The plecotine genera are related to Myotis as evidenced by the possession of all four biarmed chromosomes found in that genus. The occurrence of the same small biarmed chromosome in Idionycteris, Euderma (11), Plecotus (10), and Myotis (16/17 of Bickham) clearly unites the plecotine genera with the Myotis group in contrast to the opinion of Williams et al. (1970). This chromosome is believed to be the result of an inversion in an acrocentric chromosome of the original vespertilionid karyotype of 50 or more telocentric or acrocentric chromosomes as seen in the genus Eptesicus. The data derived from these comparisons are presented cladistically to make the information more accessible to systematic zoologists.