Physical mapping of the 18S ribosomal RNA gene (rDNA) was carried out by fluorescent in situ hybridization (FISH) in the endangered freshwater fish O. belangeri. The specimens were collected from Imphal valley, Manipur, India and metaphase chromosome preparation was made using standard hypotonic treatment, methanol-acetic acid fixation and flame-drying technique followed by Giemsa, silver, CMA3 staining for complete cytogenetic characterization of the species. The diploid chromosome number was found to be 50 and the karyotype composed of 6m+16sm+12st+16t (FN=72). One active rDNA site, located on short arm of 3rd submetacentric chromosome, was mapped by FISH and confirmed by silver and CMA3 staining. The karyomorphology, chromosomal location of rDNA loci in this species and the utility of these cytogenetic markers have been discussed in the paper.