The first chromosomal characteristics of nucleolar organizer regions (NORs) of the clown knife fish, Chitala ornata (Gray 1831) from the Chi River in Roi-Et Province, northeast Thailand, were studied. Blood samples were taken from 4 male and 4 female fish. Standard whole blood lymphocytes were cultured at 27°C for 96 h in the presence of colchicines. Metaphase spreads were performed on microscopic slides and then air-dried. Conventional and Ag-NOR staining techniques were applied to stain the chromosomes. The results showed that the diploid chromosome number of C. ornata was 2n=42, and the fundamental number (NF) was 44 in both male and female. Chromosomes types were present as 8 large telocentric, 2 medium acrocentric, 14 medium telocentric, and 18 small telocentric chromosomes. No strange sized chromosomes related to sex were observed. The region adjacent to the short arm near the centromere of chromosome pair 11 showed clearly observable secondary constriction/NORs. The karyotype formula for C. ornata is as stated:
2n (diploid) 42 = L^t₈ + M^a₂ + M^t₁₄ + S^t₁₈