A search of the recently completed genomic database of rice (Oryza sativa) identified a 29-member xyloglucan endotransglucosylase/hydrolase (OsXTH) gene family. This first report of a complete XTH family from a monocotyledonous species reveals that the OsXTH family is comparable in size with that of the dicotyledon Arabidopsis thaliana, which consists of 33 AtXTH genes. This is surprising because xyloglucan, the specific substrate of XTHs, is considerably less abundant in cell walls of monocotyledons than dicotyledons and is not typically ascribed an important structural role in monocotyledons. As a first step toward determining the roles of rice XTHs, the expression patterns of all 29 OsXTH genes were examined using a quantitative DNA microarray procedure with gene-specific oligonucleotide probes. The analysis showed that most members of the rice XTH family exhibited organ- and growth stage-specific expression. This was confirmed by quantitative real-time reverse transcriptase-polymerase chain reaction analysis of representative OsXTH members. This revealed in more detail the temporally and spatially controlled expression profiles of individual OsXTH genes at particular sites in rice. Previous reports indicated that grasses have relatively greater xyloglucan endotransglucosylase activities, one of the two enzyme activities catalyzed by XTHs, than in equivalent tissues in dicotyledons. This observation, together with the tissue-specific and growth stage-dependent expression of a large rice XTH gene family, suggests that xyloglucan metabolism plays a more central role in monocotyledon cell wall restructuring than has been reported previously.