Citrus tristeza disease caused by citrus tristeza virus or CTV, one of the mostdestructive diseases recognized worldwide, has been conducted on economical importantcitrus species grown in Thailand since October 1998. With specific emphasize on citrus species namely mandarin (Citrus reticulata), sweet orange (C. sinensis),ummelo (C. grandis), small acid seedy lime (C. aurantzfolia), leech lime (C. ystrix), and calamondin (C. madurensis), five types of disease symptoms were observed. Stempitting (SP), vein clearing (Vcl), leaf mottling (LM), leaf cupping (LC), vein corking (VC), and no symptom (NS) or symptomless were notified as symptom expressionson those citrus species. One hundred and thirty-one citrus field samples were collected and studied for the present of CTV. All diseased appearance samples were found to contained thread-like virus particles, approximately 10-12 rim x 2,000 rim, which resembling those of CTV in the Closteroviridae family. From thirtynine samplesof no symptom, twenty-nine samples revealed the CTV particles. No symptom, veinclearing, leaf cupping, stunting, mind stem pitting symptoms were induced onlime seedlings by different CTV isolates after bud-inoculation for symptom detection. For comparison methods, the dsRNA analysis and RT-PCR amplification were applied. A consistent strong band of dsRNA fragment, 13.3 x 10('6) Da MW, approximately twice the size of genomic ssRNA of CTV, was found from diseased samples. PCR-amplified fragments approximately 670 by were recovered from all diseased samples by using a pair of primers C74 and C 100 for CTV-CP gene amplification. The use of dsRNA analysis and RT-PCR technique were demonstrated as alternative fast and effective diagnostic tools for CTV detection. According to the isolation and cloning CTV-CP gene of selected field isolates, a pair of designed forward primer (CTV-Bam) (5'GGATCCGACGAAACAAAGAAATTGAAG 3') and reverse primer (CTV-Hind) (5'AAGCTTTCAACGTGTGTTGAATTTC 3') was used for the RT-PCR of the CTV-CP gene. The CTV-CP nucleotide sequences were analyzed and compared to the CTV-Florida decline isolate T 36. Moreover, the 678 by fragment of CTV-CP isolate MK50contained a full-length CP of 666 by including 12 by of ~iBam~iHI and ~iHind~ilIl linkers wassuccessfully constructed into expression plasmid vector (pQE 30) and transformed into ~iE.coli~i Ml5 host strain for protein expression. The recombinant CTV-CP protein, size of Mr - 25 KD, was purified through an affinity chromatography using a Ni-NTA super-flow resin column. Purified recombinant protein was dialyzed in PBS and mmunized in rabbit and hen for antibodies production. Antiserum to recombinant CTV coat protein was also recovered and determined by ELISA.