The nucleocapsid (NP) of wheat rosette stunt virus (WRSV) was used to separate L, NS and N proteins. By two-step dissociation of the NP particles and subsequent ultracentrifugation through a gly- cerol cushion, four viral fractions were obtained: L protein, NS-N-RNA complex, NS protein and N- RNA complex. All these fractions did not demonstrate RNA polymerase activity in vitro when assayed individually. After various recombinations of these fractions, we found that the RNA polymerase activity was reconstituted only when the L, NS and N-RNA complex were all present in the assay mixture. Since the dissociable L and NS proteins were both required for the RNA synthesis in vitro, we presume that they probably constitute together the RNA polymerase complex. The fact that N-RNA complex could not be replaced by phenol extracted RNA as template for the in vitro transcription, and the tight association of N protein with the viral RNA suggests that the N protein associated with the RNA may function as a factor which makes the RNA form active template for the transcription.