Sucrose gradient centrifugation of heat-denatured RNA of turnip yellow mosaic virus permitted the isolation of five RNA classes with molecular weights ranging from 2.0 to 0.25 X 10(6). The infectivity was shown to be confined to an RNA molecule of molecular weight 2.0 X 10(6). No significant increase in infectivity was obtained by combination of the latter RNA with the RNA classes of smaller size. Translation in vitro of the RNAs of different size classes in a wheat germ cell-free system revealed that the infectious RNA (molecular weight 2.0 X 10(6) does not promote the synthesis of the coat protein of turnip yellow mosaic virus. Efficient production of this coat protein was found exclusively when the smallest RNA class (molecular weight 250,000) was used as a messenger. It is concluded that RNA molecules of turnip yellow mosaic virus of molecular weight 2.0 X 10(6) contain a closed coat protein cistron and that RNA molecules of molecular weight about 2 to 3 X 10(5) with an open coat protein cistron can be isolated from the virions.