A cDNA clone ($Zds$) encoding zeta-carotene desaturase (ZDS) was isolated from the $Citrus$ fruit cDNA library. Sequence analyses indicated that the clone contains an open reading frame encoding 570 amino acids. A genomic Southern blot analysis and phylogenic dendrogram indicated that the $Zds$ gene is present as more than two isoforms in the $Citrus$ genome. Sequence analyses revealed that the cDNA showed significant similarity to other dicotyledonous woody plants. An RNA blot analysis showed that the transcripts were expressed in all investigated tissues of $Citrus$. During the ripening of fruits, the signal of the $Zds$ transcripts was very weak until the break stage, but its expression markedly increased to the maximum level at the last stage. A similar pattern was detected in peels of citrus fruits. Furthermore, the level of $Zds$ transcripts was changed in the process of leaf development. The results suggest that $Citrus$ Zds is one of the important regulatory enzymes in carotenoid accumulation especially in the last stage of fruit ripening.