Technical Abstract: Cotton fibers are unicellular trichomes originating from the ovule epidermis. Fiber cells elongate rapidly bounded by a thin primary wall until secondary wall synthesis begins. Cytoskeleton assemply plays an imporotant role in determining fiber cell length and morphology and is developmentally regulared. As in other plant cells, it is not clear how cytoskeletal assembly in fibers is regulated. Recently, several Rac/Rop GTPases in Arabidopsis were shown to regulate isotropic and polar cell growth of root hairs and pollen tubes by controlling assembly of the cytoskeleton. RhRac1 isolated from cottonseeds is a member of the Rac/Rop GTPase family and is abundantly expressed in rapidly growing cotton tissues. GhRac1 shows the greatest sequence similarity to the group IV subfamily of Arabidopsis Rac/Rop genes. Over-expression of GhRac1 in E coli led to the production of a functional GTPase. In contrast to other Rac/Rop GTPases found in cotton fiber, GhRac1 is highly expressed during the elongation stage of fiber development with expression decreasing dramatically when the rate of fiber elongation declines. The association of higihest GhRac1 expression during stages of maximal cotton fiber elongation suggests that GhRac1 GTPase may be a potential regulator of fiber elongation by controlling cytoskeletal assembly.