Full length cDNAs of Cucumber mosaic virus (CMV) CB7 strain, causing necrosis on Nicotiana glutinosa, were obtained by RT-PCR, using viral genomic RNAs as templates. cDNAs of CMV-CB7 genomic RNAs were cloned and sequenced and results indicated that RNA1, 2 and 3 was 3 356 nt, 3 045 nt and 2 218 nt, respectively (accordingly Accession Number EF216866, DQ785470 and EF216867). Infectious RNA transcripts from cDNA clones of CMV-CB7 were inoculated onto N. glutinosa and the seedlings of host plants displayed necrosis symptom, whist that of CMV-Fny induced typical mosaic symptoms. Through pseudorecombination between CMV-CB7 and CMV-Fny genomic RNAs, the genetic determinant of necrosis phenotype was mapped to RNA2. Chimeric infectious clones consisting of partial sequences of RNA2 derived from CMV-CB7 and CMV-Fny, respectively, were obtained by Overlapping PCR. Pathogenic analysis with those chimeric RNA2 revealed that 2b gene or 3′UTR of CMV-CB7 RNA2 was responsible for the necrotic pathotype. Northern blotting analysis reflected that both necrotic and non-necrotic viruses accumulated to similar levels of genomic RNAs in host plants. Therefore, necrotic phenotype induced on N. glutinosa was not related to the level of accumulation of CMV genomic RNAs.