Karyological investigations of two JapaneseNyctalusspecies were made based on G- and C-banding pattern analysis. The karyotype ofN. furvus(2n= 44, FN = 50) was similar to the standard karyotype ofMyotisspecies. On the other hand, the karyotype ofN. lasiopterus aviator(2n= 42, FN = 50) was characterized by having an additional large metacentric pair. By comparison of the G-banding pattern between the twoNyctalusspecies, it is clear that the metacentric pair ofN. lasiopterus aviatorhas derived from the Robertsonian fusion of the two pairs (nos. 8 and 10) ofN. furvus.There was also a significant difference in amount of constitutive heterochromatin between the twoNyctalusspecies. InN. furvusthe additional heterochromatin was observed on the majority of autosomes, while inN. lasiopterus aviatora small C-band was restricted to the centromeric area of each autosome. Thus, the C-band pattern ofN. lasiopterus aviatorwould be interpreted by assuming the decrease of heterochromatin in the course of its speciation fromN. furvus.