Partial sequences of a number of cDNAs showing homology with WRKY genes were amplified from two-day cold-acclimated Poncirus trifoliata by reverse transcription-polymerase chain reaction (RT-PCR) using degenerate primers designed based on conserved signature sequences of the WRKY domain. The full-length sequence of one of these cDNAs designated as PtrWRKY2 was obtained using rapid amplification of cDNA ends (RACE) method. PtrWRKY2 cDNA was 2,070 bp in length containing an open reading frame (ORF) encoding a polypeptide of 540 amino acids. The polypeptide was homologous to WRKY transcription factors (TFs) having double WRKY domains with Cys(2)His(2) signature motif. The PtrWRKY2 gene was clustered with putative WRKY TFs having double WRKY domains and showed close phylogenetic relation with putative WRKY TFs mostly from woody perennial plants. The expression of the PtrWRKY2 gene was analyzed by Northern blot in cold-hardy Poncirus and cold-sensitive Citrus species, Citrus grandis (pummelo) having the same gene with more than 95% nucleotide sequence homology. The results revealed that the expression of the PtrWRKY2 gene was initially induced in response to cold in both cold-hardy Poncirus and cold-sensitive pummelo; however, the expression was decreased at 1 h of cold acclimation and then, clearly repressed in Poncirus and pummelo. On the other hand, drought stress had no effect on the expression of the PtrWRKY2 gene in pummelo, but its expression was repressed by drought stress in Poncirus indicating a negative regulation. The results showed that PtrWRKY2 encodes a WRKY TF and its expression showed differential responses to cold acclimation and dehydration stress in Poncirus and Citrus.