Members of the plant-specific gene family referred to as the NAC family (for NAM-ATAF-CUC-related) are involved in various functions including the regulation of plant development. However, no detailed molecular characterization of any member of the NAC family has yet been reported from monocots. Here, we report such a characterization of ONAC300, a novel NAC-family gene identified using a cDNA cloned from microdissected phloem cells of rice. The predicted ONAC300 protein sequence falls into the NAM subgroup, which also contains the proteins CUC1 and CUC2 from Arabidopsis, CUP from snapdragon, CmNACP from pumpkin and NAM from petunia. High levels of ONAC300 mRNA were detected by in situ hybridization in developing shoot apical meristem (SAM) and in the associated young leaves. The use of an ONAC300:: GUS reporter gene revealed that the ONAC300 promoter was expressed predominantly in developing vascular tissues of the leaves and roots. The construct was also expressed in anther filaments, rachis and carpel styles. RT-PCR analysis further revealed that the levels of ONAC300 transcripts were higher in leaves, roots and culms than in panicles. The observed expression pattern of ONAC300 is quite different from those of the dicot NAC genes previously reported. Thus, ONAC300 is a novel member of the NAC family which is expressed at very early developmental stages in the shoot, root and flower, as well as in the mature phloem of vascular tissues in rice.