The spatial and temporal distribution of sucrose synthase (RSuS) in rice (Oryza sativa L.) was studied by Western and immunohistochemical analyses using the monospecific antibodies for three RSuS isoforms. In leaf tissues, RSuS1 was localized in the mesophyll while RSuS2 was in the phloem in addition to the mesophyll. In the roots, only RSuS1 was found in the phloem. No RSuS3 could be detected in any parts of etiolated seedlings. The expression of each RSus gene is closely linked to the seed development. RSuS1 was present in the aleurone layers of developing seeds, and at a low level in endosperm cells. RSuS2 was evenly distributed in seed tissues other than the endosperm. RSuS3 was localized predominantly in the endosperm cells. The tissue specific localizations of the three gene products suggest that RSuS1 plays a role in sugar transport into endosperm cells where the reaction catalyzed by RSuS3 provides the precursor of starch synthesis. RSus2, which is ubiquitously expressed, may play a housekeeping role.