WRKY transcription factors (TFs) are involved in regulation of abiotic and biotic stress responsive genes and play a role in abiotic and biotic stress tolerance in plants. Although commercial Citrus species are sensitive to cold and drought stresses, an interfertile Citrus relative, Poncirus trifoliata, is resistant to cold and more drought tolerant than most commercial Citrus cultivars. An expressed sequence tag clone sharing homology with WRKY-type TFs was previously isolated from a cDNA library constructed using two-day cold-acclimated P.trifoliata cv. Rubidoux seedlings. In this study, a full-length sequence of this WRKY gene was cloned and isolated from 2-day cold-acclimated P. trifoliata plants using the rapid amplification of cDNA ends (RACE) method. The full-length PtrWRKY1 cDNA is 1807 bp and encodes a polypeptide of 405 amino acids containing a single WRKY domain with a Cys(2)His(2) motif, which is a signature of WRKY TFs. A homologous open reading frame (ORF) sequence was isolated from Citrus grandis, CgWRKY1, sharing 98% identity at the nucleotide level with PtrWRKY1. The expression of PtrWRKY1 and CgWRKY1 was investigated in response to cold and drought stresses by northern blot analysis. While the expression of both genes was induced in response to drought, only PtrWRKY1 expression was induced in response to cold. This differential expression in cold-hardy Poncirus and cold-sensitive pummelo suggests that PtrWRKY1 gene may play a role in adaptation and tolerance to cold stress in Poncirus.