Ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) from a red alga, Galdieria partita, has been crystallized by the hanging drop vapor diffusion method. Two forms (Forms I and II) of crystals were obtained under distinct conditions. The Form I crystal belongs to monoclinic space group C2 with cell dimensions of a = 190.2, b = 140.0, c = 189.0 A, and beta = 102.6 degrees, and diffracts up to 3.0 A resolution. Diffraction from the Form II crystal was too weak to determine crystal data.