Solid-state properties of active pharmaceutical compounds are closely related to their dissolution behavior and bioavailability, and understanding their phase behavior leads to better control in the drug formulation process. L-Citrulline is an amino acid, which is known to exhibit polymorphism; however, only the structure of the alpha form is reported in the literature. The structure of the form has been elusive due to lack of good quality single crystals. It has been found to be orthorhombic P2(1)2(1)2(1) with the cell parameters a, b, and c respectively 14.895(5) angstrom, 9.852(2) angstrom, and 5.353(2) angstrom. The unit cell contains four molecules, and its volume is 785.5(4) angstrom(3). Both the alpha form and the delta form possess uniaxial negative thermal expansion along the direction of the charge exchange of the L-citrulline zwitterions. In mixtures containing solid and saturated aqueous, solution, the alpha form quickly transforms in a dihydrate, whereas the delta phase persists for over three weeks. Although the evidence is not conclusive, the delta phase is most likely the more stable form and most suitable for storage.