A virus-induced, viral RNA-specific, RNA-dependent-RNA-poly-merase activity has been observed in vitro associated with membrane extracts of plants of three different species after infection with grapevine chrome mosaic nepovirus (GCMV). The products of this activity are full-length, positive sense GCMV RNAs present in double-stranded structures. As has been demonstrated for a relative to nepoviruses, cowpea mosaic comovirus, this activity probably corresponds to the nepoviral replication complex.