Plant Rac proteins belong to an important group of signal switches anchoring on membranes, involved in various physiological processes including cell polar growth, synthesis of secondary wall, resistance response and hormone signaling. In the attempt to elucidate the molecular mechanism of initiation and elongation of cotton fiber, two cotton Rac protein genes, designated as GhRacA and GhRacB, were amplified from elongating fibers and cloned. It was demonstrated that, the cDNA of GhRacA contained 959 bp and encoded a putative polypepetide of 211 aa, while GhRacB was 920 bp in length, encoding a predicted protein of 195 aa. These two cotton Rac proteins, GhRacA and GhRacB, contained conserved regions involved in GTP/GDP binding and activation, an effector region and a polybasic region. A conserved prenylation site CSIL was found in GhRacB, while no apparent prenylation site was discovered in GhRacA. Sequence comparisons showed that GhRacA and GhRacB were two novel Rac proteins from cotton. The expression patterns of GhRacA and GhRacB was analyzed by RT-PCR. It was demonstrated that these two Rac protein genes were both expressed in root, hypocotyls, stem, leaf and fibers, and the highest level of transcripts was to accumulate in the fibers at the stage of initiation and elongation, suggesting that the two Rac genes, GhRacA and GhRacB, might play an important role in the early stage of fiber development.