Double-stranded RNA (about 14 kb) is found in all cultivated rice (Oryza sativa) and wild rice (O. rufipogon) tissues. It is detected at every developmental stage, and is transmitted very efficiently to progeny via seeds (more than 98%). The dsRNA is maintained at a constant level (approximately 100 copies/cell) in almost all tissues. However, the number of copies increases about 10-fold when host cells are grown in suspension culture. Complete nucleotide sequences of cultivated rice (temperate japonica rice, cv. Nipponbare, J-dsRNA) and wild rice (W-1714, W-dsRNA) dsRNAs have been determined. Both wild and cultivated rice dsRNAs have a single long open reading frame (ORF) containing the conserved motifs of RNA-dependent RNA polymerase and RNA helicase. The coding strands of both contain a site-specific discontinuity (nick) at nt 1,211 (J-dsRNA) or at nt 1,197 (W-dsRNA) from the 5 end of their coding strand. Rice dsRNA has several unique properties and can be regarded as a novel RNA replicon. This paper discusses the origin and evolution of the rice dsRNA.