We identify a gene, organ boundary1 (OBO1), by its unique pattern of enhancer- driven GFP expression at the boundaries between the apical meristems and lateral organs in Arabidopsis embryos, seedlings, and mature plants. OBO1 also is expressed at the root apical meristem and in distinct cell files surrounding this area. OBO1 is one of a 10-member plant-specific gene family encoding a single small domain (133 amino acids) with unknown function. One member of this gene family, OBO2, is identical to a previously studied gene, light-sensitive hypocotyl1. Overexpression of OBO1 causes an abnormal number and size of petals and petal-stamen fusions. The patterns of OBO1 gene expression are distinct but overlap with other genes involved in boundary formation in the Arabidopsis shoot apical meristem, including cup-shaped cotyledon, lateral organ boundaries, blade-on-petiole, asymmetric leaves, and lateral organ fusion. Nuclear localization of OBO1 suggests that it might act with one or more of the transcription factors encoded by the foregoing genes. Ablation of the specific cells expressing OBO1 leads to loss of the shoot apical meristem and lateral organs. Thus, the cells expressing OBO1 are important for meristem maintenance and organogenesis in Arabidopsis.