A novel mutation affecting the lateral symmetry of rice leaves, leaf lateral symmetry 1 (lsy1), was identified. The lsy1 mutant showed two major phenotypes of leaves; a narrow leaf due to the deletion of a part or whole of one lateral half, and a bifurcated leaf in which an additional midrib is formed in one lateral half. Developmental analysis revealed that in the leaf primordia of the latter type, an additional protrusion, which would later develop to a midrib, was formed in the center of one lateral half, suggesting that the lateral half of the primordium was developing as a single leaf. Other phenotypes associated with leaf lateral symmetry were also observed. Thus, in lsy1 leaves, the two lateral halves did not grow synchronously. Lateral asymmetry was also observed in leaf-like spikelet/floral organs such as glumes and carpel. The LSY1 gene is considered to play an important role in establishing lateral symmetry of leaves and leaf-like organs.