All chromosomes of rice have submedian centromeres except Chromosomes 4 which has a submedian centromere. The three largest chromosomes showed two such constrictions, one on each arm. Haploid mitotic chromosomes showed the existence of four pairs of chromosomes with similar size and morphology. The largest four chromosomes could not be paired on this basis. Chromomere pattern at pachytene for all chromosomes were distinct. However, the chromomeric heterochromatin appeared frequently to resolve into smaller units. One or more euchromatic gaps of various sizes were observed for most chromosomes. These were terminal or interstitial and produced smaller nucleoli at various times in development. Up to four pairs of bivalents were found to have similar chromomeric patterns for corresponding to the mitotic findings. Chromosome 9 was found to be the nucleolar organizing chromosomes with a smaller nucleolar chromosome (Chromosome 10) to support it. It also showed a 'collar' configuration which could be explained by a reverse tandem duplication involving two chromosomes. Pachytene in the haploids revealed complete pairing between most of the supposingly univalent chromosomes thus confirming the presence of the duplicate chromosomes. Maximum pairing at pachytene was four complete bivalents plus a quadrivalent comprising the four largest chromosomes. Further diploid pachytene analysis of Chromosomes 1 & 3 and 2 & 4 to have similar chromomeric patterns indicating that these two pairs of chromosomes were originally the same in size and structure. Karyotype analysis thus strongly indicates O. sativa L. to have an autotetraploid origin derived from a n=6 chromosome ancestor which has undergone further differentiation in its chromosome and a complete chromosomal diploidizing through strict regulation of pairing into bivalents. A karyotype idiogram and a chromomeric map have been prepared.