Cotton fibers (Gossypium hirsutum L.) are highly elongated trichomes that grow from epidermal cells of cotton ovules. Analysis of genes that are specifically expressed during fiber differentiation can provide insights into the molecular events that control cotton fiber development. One such gene, called Gh-1, has been isolated and characterized. Deletion analysis of the 5' flanking sequences of the Gh-1 gene was performed to identify the putative regulatory elements responsible for fiber specific expression. The Gh-1 promoter, along with various lengths of 5' flanking sequences, were fused with the b-glucuronidase (GUS) reporter gene and transferred into tobacco and cotton plants via Agrobacterium-mediated transformation. Tobacco plants that carry the Gh-1::GUS gene construct with 1010 bp of 5' flanking sequence (-1010) from the first AUG start codon typically showed very weak GUS activity in vascular tissues and guard cells, and, in some plants, expression was seen in small glandular trichomes. Similar expression patterns were also seen in plants that carried -780, and -680 Gh-1::GUS promoter constructs. A dramatic increase in specific expression in large glandular trichomes was observed in plants that carried a -550 Gh-1::GUS promoter construct. Promoter deletion constructs -355 Gh-1::GUS or shorter showed no observable GUS staining. In transgenic cotton, promoter deletion -1010 Gh-1::GUS construct showed no observable GUS staining in stem and 0-3 DPA ovules. However, promoter deletion -550 Gh-1::GUS showed strong GUS staining in guard cells, glandular trichomes in the stem, 0-3 DPA ovules, and 20 DPA cotton fibers. These results indicated that a trichome and guard cell "specific" regulatory element apparently exist between -550 and -355 of the Gh-1 promoter.