Karyotype analysis and comparison of chromosome size in four Oryza species, O.sativa, O.perennis, O.punctata and O.officinalis, possessing three different genomes separately, were carried out. As a standard, chromosomes in late prophase, prometaphase and early metaphase cells of O.sativa were analysed. Stages were discrirninated with chromosome length and then the relative length and the arm ratio were measured in three stages. It appeared that the order of chromosome length and the position of centromeres were much the same in all stages, though the staining pattern changed as the stage advanced. In each stage, however, individual chromosomes constantly showed their own staining pattern. Therefore, chromosome identification could be done easily. Karyotypes of other three species were much the same as O.sativa. Characteristics in relative length, arm ratio and staining pattern of twelve chromosome pairs were very similar among four species. In addition, no difference was detected in chromosome size among four species, that is, the absolute length and the ratio of width to length were almost constant in a given stage of these species. So, we concluded that the chromosomes of four species had not been much differentiated morphologically each other.
