This study deals with the cloning and characterization of monosaccharide transporter cDNAs in rice. OsMST1-3 (Oryza sativa monosaccharide transporters 1-3) have two sets of putative six transmembrane domains separated by a central long hydrophilic region. Heterologous expression of OsMST3 in the yeast Saccharomyces cerevisiae indicated that OsMST3 has transport activity for some monosaccharides in an energy-dependent H+ co-transport manner. Northern blot and in situ hybridization analyses showed that OsMST3 mRNA is detectable in leaf blades, leaf sheaths, calli and roots, especially the xylem as well as in sclerenchyma cells in the root. These results suggested that OsMST3 is involved in the accumulation of monosaccharides required for cell wall synthesis at the stage of cell thickening.