OsSUT1 encodes a rice sucrose transport protein that is highly expressed in developing grain, leaf sheath and stem after heading, and in germinating seedlings, but only at very low levels in source leaves. In this study, we have used antisense gene suppression to elucidate the in vivo function of OsSUT1. Rice was transformed with an antisense construct containing a portion of the 3′-coding and non-coding regions of OsSUT1 driven by the maize ubiquitin-1 promoter. Twenty-six independent stably transformed lines were obtained. T0 and selfed T1 progeny were analysed for suppression of OsSUT1 expression and function. Many of the plants showed a significant reduction in their ability to produce filled grain, and final grain weight was reduced. Severe phenotypes correlated with a reduction in OsSUT1 transcript level in filling grain. Unlike SUT1 antisense suppression in dicots, source supply of photosynthate was unaffected in these transformants. This provides the first direct evidence for the requirement of a sucrose transporter for grain filling in a cereal species. Furthermore, seed from some of the T0 population showed a reduction in the rate of germination and growth, supporting the hypothesis that OsSUT1 may also play a role in transporting sucrose remobilized from starch reserves in germinating seeds.