Two satellite RNAs were found in cucumber mosaic virus strain Y (CMV-Y) propagated in Nicotiana tabacum cv. Ky 57 in our laboratory; a larger molecular weight satellite RNA (Sat I) and a smaller one (Sat II). In virus from Ky 57 tobacco leaves which had been inoculated with genomic RNAs (RNAs 1-3) of the CMV, the satellite RNAs were not detected. This virus we termed CMV (-Sat). Symptoms on Ky 57 tobacco infected with CMV (-Sat) or with CMV (-Sat)+the satellite RNA(s) (Sat I or/and Sat II) were similar. However, addition of the satellite RNAs reduced yield of the virus by about half. The virus purified from tobacco (N. tabacum cvs. Ky 57 and Xanthi nc) leaves inoculated with CMV (-Sat)+Sat I contained only Sat I even after successive passages of the virus in tobaccos. The virus purified from tobaccos inoculated with CMV (-Sat)+Sat II contained both Sat I and Sat II, and Sat I became predominant after repeated passages of the virus in tobaccos. When tomato (Lycopersicon esculentum cv. Fukuju-nigo) was inoculated with CMV (-Sat), ordinary mosaic symptoms appeared with no necrotic symptoms. When the tomato was inoculated with CMV (-Sat)+the satellite RNAs, lethal necrosis appeared. The lethal necrosis was more severe when CMV (-Sat) was mixed with Sat I than when mixed with Sat II, and in these cases virus purified from the tomato expressing lethal necrosis symptoms contained only Sat I. Thus, it was suggested that Sat I was primarily involved in the tomato necrosis syndrome.