The present study reports the first finding of the NOR (nucleolar organizer region) polymorphism and chromosome analysis in the John's snapper (Lutjanus johnii) from Thailand. Kidney cell samples were taken from four male and four female fish. Mitotic chromosome preparations were conducted using the standard squash technique, as well as directly from kidney cells. Metaphase spreads were performed on microscopic slides and then air-dried. Conventional and Ag-NOR banding techniques were applied to stain the chromosome. The results showed that the diploid chromosome number of L. johnii was 2n=48, and the fundamental number (NF) was 48 in both male and female. Karyotpes were present as 28 large telocentric, 16 medium telocentric, and 4 small telocentric chromosomes. No strange-sized chromosomes related to sex were observed. The results indicated that the long arm subcentromeric of the telocentric chromosome pair 1 showed clearly observable NORs (satellite chromosomes). In addition, a heteromorphism of one female had different-sized NORs in chromosome pair 1 (1a1b), while three females and four males had equal-sized NORs in chromosome pair 1 with a homomorphism (1a1a). The karyotype formula for L. johnii is as follows:
2n (diploid) 48=L^t₂₈+M^t₁₆+S^t₄