Nucleolar organizer regions (NORs) and karyotype in the black lancer, Bagrichthys majusculus from Nakhon Phranom and Sing Buri provinces, Thailand, were investigated. The mitotic chromosome preparations were directly obtained from kidney tissues of eight male and eight female specimens. Conventional and Ag-NOR staining techniques were applied to the chromosomes. The chromosomes number of B. majusculus was 2n=50 and the fundamental number (NF) of all specimens was 96. The karyotype consists of six large metacentric, six large submetacentric, two large acrocentric, eight medium metacentric, 12 medium submetacentric, two medium acrocentric, six small metacentric, four small submetacentric and four small telocentric chromosomes. Sex chromosome was not observed. Moreover, the interstitial NORs were clearly observed at the long arm of the metacentric chromosome pair 1. The result revealed that a heteromorphic NOR type was appeared in one male and one female fish and other fish have homozygous NOR in the pair 1 chromosomes.