Leguminous plants can fix and assimilate atmospheric nitrogen by root nodules as symbiotic organs with soil bacterium rhizobia. In soybean (Glycine max [L.] Merr.), it is well known that nodulation is systemically suppressed by the rapid response to the initial rhizobial infection. Nodule number is essentially controlled by host plant because substantial amounts of photoassimilates are required for nodule growth and the maintenance of nitrogen fixation including nitrogen assimilation and transport. The negative regulating mechanism is referred to as “autoregulation of nodulation” , which is controlled by shoot through the exchange of unknown signal molecule(s) between shoot and root (Olsson et al., 1989; Caetano-Anolles and Gressoff, 1991; Francisco and Harper, 1995; Sato et al., 2002). About 7-8 days after inoculation, nodules initially appear on the roots. The early stage of nodule formation is important to determine the development of infection sites, because the infection sites are destined by their fate to develop into nodules or remain in the stage just after infection followed by cell division. Since the 1980s, hypernodulation mutants of soybean have been isolated from several soybean cultivars following chemical mutagenesis (Carroll et al., 1985; Gremaud and Harper, 1989; Akao and Kouchi, 1992). The fact that hypernodulation phenotype is decided by shoot genotype of hypernodulation mutant, not root genotype; hypernodulation mutants lack or decline a part of the communication between shoot and root of autoregulation of nodule formation. Plant growth of hypernodulation mutant is less vigorous than wild type (Carroll et al., 1985; Gremaud and Harper, 1989; Akao and Kouchi, 1992); however, it is unclear that inferior growth is appeared by gene mutation or secondary effect of hypernodulation trait. I t i s we l l known that substant ia l amounts o f photosynthate are required for nitrogen fixing activity in mature nodule (Fujikake et al., 2003). In the cv. Williams, we reported that photosynthate allocation for nodule formation was not markedly increased during the early stage; the inoculated roots gradually had priority of photosynthate allocation after the emergence of nodules at day 10 after inoculation (Ito et al., 2006). The objective of this study is to clarify how is the photosynthate allocation in hypernodulation mutant. So we investigated current photosynthate allocation f o r nodu l e f o rma t i on dur ing the ea r l y s t age i n hypernodulation mutant NOD1-3, which is isolated from cv. Williams (Gremaud and Harper, 1989).