Cold is the major environmental limitation to crop productivity. Citrus species are vulnerable to freeze injury at -2.2°C and below. Yet some of the most valued citrus crops are grown in relatively high-risk freeze areas. The Texas citrus industry lost about 165,000 acres in consequence of the freezes of 1951, 1962 and 1983, and became almost unsustainable after the 1989 freeze. The Florida citrus industry was forced to relocate to warmer southern areas of the State in an attempt to minimize the impact of the deadly freezes. Poncirus trifoliata (trifoliate orange) is a citrus relative that can withstand temperatures as low as -16oC after being acclimated, and it is an excellent source of genes for tolerance to this important abiotic stress. Since cold tolerance is known to be controlled by several genes, the objective of this research was to identify genes that are up regulated by cold. This approach will lead to the identification and characterization of important cold-induced genes that in the future can be introduced into susceptible crops in order to enhance freezing tolerance. Comparison of gene expression of trifoliate orange plants exposed to 28oC with those gradually acclimated to -1oC was performed. Ten cDNA fragments up regulated in plants acclimated to -1oC were isolated and sequenced. These fragments showed similarities at the amino acid level to genes with known cold related functions such as transcription factors, transcriptional regulators, zinc finger containing proteins, and calcium regulated channels.