We report the first chromosome analysis and localization of the nucleolar organizer region of the Phayre's flying squirrel, Hylopetes phayrei (Blyth, 1985) from Thailand. After standard peripheral blood lymphocytes had been cultured at 37°C for 96 h in the presence of colchicine, the metaphase spreads were performed on microscopic slides and air-dried. Conventional staining, GTG-banding, high-resolution and Ag-NOR staining techniques were applied to stain the chromosomes. The results showed that the diploid chromosome number of H. phayrei was 2n = 38 and the fundamental number (NF) were 76 in both males and females. The karyotype has the presence of 2 large metacentric, 8 large submetacentric, 8 large acrocentric, 6 medium acrocentric, 6 small metacentric and 6 small acrocentric chromosomes. The X chromosome is a small submetacentric chromosome and the Y chromosome is a small acrocentric chromosome. From the GTG-banding and high-resolution techniques, the numbers of bands and locations in the H. phayrei are 182 and 234, respectively; each chromosome pair could be clearly differentiated. The Ag-NOR staining demonstrated that the three pairs of nucleolar organizer regions (NORs) were observed on the long arm regions of chromosome pairs 1, 5, and 14. The karyotype formula of H. phayrei could be deduced as:
2n (diploid) 38 = L₂^m+L₈^sm+L₈^a+M₆^a+S₆^m+S₆^a+sex-chromosomes