Abstract: By using specific primers designed on the basis of conserved sequences of coat protein gene,a multiplex PCR protocol for the detection of 3commonly found disease-causing viruses,i.e.,Cymbidium mosaic(CyMV),Odontoglossum ringspot(ORSV)and bean yellow mosaic(BYMV),in Oncidium was developed.Three specific fragments were simultaneously amplified in a single PCR reaction system.Their molecular weights were determined to be 571 bp for CyMV,325 bp for ORSV,and 212 bp for BYMV.Sequence analysis indicated that the 3viruses were at least 98% homogenous with their respective reference viruses.The sensitivity test on this newly developed methodology showed that it was capable of accurately detecting the 3viruses simultaneously in a bulbous iris tissue at the level of 10-2 mg.