Oligonucleotides for a conserved region of the coat protein gene of cucumber mosaic virus (CMV) and a hammerhead structure ribozyme against CMV RNA were synthesized using a DNA synthesizer. Both strands of oligonucleotides were annealed and restricted with BamHI/SacI, then cloned into a plasmid pBS SK (+). The cloned CMV substrate and ribozyme were sequenced to verify correct constructions. In vitro transcriptions were carried out by using T7 RNA polymerase with BssHII or SspI digests of 1 ㎍ of substrate and ribozyme clones. The size of substrate RNA was 176 nucleotides (nt) containing 50 nt of CMV RNA sequence, 6 nt of XbaI restriction site and 120 nt of vector-derived sequence in the case of BssHII digest. The size of ribozyme RNA was 164 nt containing 40 nt of ribozyme RNA sequence and same sequences of substrate. Substrate RNA was efficiently cleaved into two fragments (96 nt and 80 nt) by ribozyme RNA. This endonucleolytic cleavage occurred more efficiently at 55℃ than 37℃. SspI digest-derived substrate RNA (2234 nt) was also cleaved into two fragments by the same ribozyme. SspI digest-derived ribozyme RNA (2222 nt) cleaved the above substrate to two fragments. In vitro-tested ribozyme construct is being cloned into a plant transformation vector to develop virus-resistant plants.