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Synthesis of turnip rosette virus (TRosV)-induced RNA was examined in turnip protoplasts inoculated in vitro. TRosV RNA (mol. wt. 1.4 × 106; approx. 4.3 kb) was first detected 10 to 20 h post-inoculation by 32P labelling. When actinomycin D was added to protoplast cultures 6 h or less post-inoculation, labelling of virus RNA was less than in untreated cultures. Synthesis of TRosV RNA in infected protoplasts appeared to involve a double-stranded (ds)RNA; a replicative form (RF) of the virus RNA was isolated and characterized. The RF, mol. wt. 2.8 × 106 (approx. 4.3 kbp), was not degraded by treatment with RNase or DNase, contained 50% sequences complementary to TRosV RNA (as determined by hybridization) and was denatured to singlestranded (ss)RNAs of mol. wt. (× 10−6) of 1.4, 0.7, 0.3 and 0.09 (4.3, 2.1, 0.9 and 0.27 kb respectively). Virus infection apparently induced the synthesis of a ssRNA of mol. wt. 0.3 × 106 which could be resolved from other RNA species in nucleic acid extracts from infected protoplasts. A minor dsRNA of 1.4 × 106 mol. wt. (2.1 kbp) was also apparently induced by virus infection. These virus-induced small RNA species may represent subgenomic messengers or their RF.