The fibre length and cellulose content are the two determinates of fibre yield and quality. The rapid elongation process and massive cellulose biosynthesis in fibre cells must depend on sufficient supply of photoassimilate. Sucrose is the predominant form of photoassimilate imported to fibres for cellulose synthesis and for generating turgor pressure to drive the fibre elongation. Sucrose and other solutes may move into fibres either symplastically through PIasmodesmata or apoplastically via the plasma membrane of the fibre cells. The cleavage of sucrose by sucrose synthase (SuSy)in the fibre generates UDPglucose, the immediate substrate for fibre cellulose biosynthesis. In developing countires, unloaded sucrose is utilized in diverse pathways for cellulose, starch and lipid biosynthesis by the fibre, seed coat and cotyledons, respectively. The strong competition by the latter two sink tissues could limitthe availability of photoassimilate for cellulose biosynthesis in the fibre, and hence reduce the fibre yield. Previous studies sugget that SuSy expression in fibres is important for mobilising sucrose into this tissue. However, conclusive evidence is lacking regarding the role of SuSy in fibre development. Little is known on the cellular pathway of import of sucrose and other solutes into the developing cotton fibre. A clear understanding of the pathway and regulation of sucrose import into fibres is essential for designing appropriate molecular approaches to enhance photoassimilate mobilization to this biosyntheticly active sink tissue for elongation and cellulose synthesis.