Suppression of brassinosteroid (BR) biosynthesis in cotton ovules by treatment with brassinazole inhibits fiber formation, indicating that BR plays an important role in cotton fiber development. Plant responses to brassinosteroids (BR) are mediated through a plasma membrane-bound leucine-rich repeat (LRR) receptor-like protein kinase known as BRI1. Mutations in the BRI1 genes of several species result in dwarfed plants with reduced sensitivity to BR. A single expressed sequence tag (EST) from cotton with strong sequence similarity to Arabidopsis BRI1 ( AtBRI1 ) was identified in a search of publicly available databases. With this EST as a starting point, full-length cDNAs and genomic coding sequences from upland cotton ( Gossypium hirsutum ) BRI1 ( GhBRI1 ) were obtained and characterized. Ectopic expression of this coding sequence in BR-insensitive Arabidopsis plants resulted in recovery of normal growth indicating that GhBRI1 is a functional homologue of AtBRI1. G. hirsutum is an allotetraploid (AADD) derived from diploid ancestors. Analysis of several GhBRI1 cDNAs showed two distinct sequences indicating the presence of two GhBRI1 genes, denoted GhBRI1-1 and GhBRI1-2. Sequence comparisons between these GhBRI1 coding sequences and those from related A and D genome diploid Gossypium species ( G. arboreum and G. thurberi ) indicated that GhBRI1-1 is likely to the A sub-genome orthologue while GhBRI1-2 is from the D sub-genome.