Genomic in situ hybridization (GISH) is a powerful tool to characterize parental chromosomes in interspecific hybrids, including the behaviour of autosynapsis and chromosome paidng. It was used to distinguish the chromosomes of Oryza sativa from wild species in a spontaneous interspecific hybdd and to investigate the chromosome pairing at metaphase I in meiosis of the hybdd in this study. The hybrid was a triploid with 36 chromosomes according to the chromosome nurnber investigated in mitosis of root tips. During metaphase I of meiosis in the hybrid, less chromosome pairing was observed and most of the chromosomes existed as univalent. Based on GISH and FISH (Fluorescent in situ hybridization) analyses, the chromosomes of the hybrid were composed of genomes A, B and C. Thus, it was believed that the hybrid was the result of natural hybridization between cultivated rice and wild species O. minuta which was planted in experimental fields.