The salt-tolerance gene SOS3 (salt overly sensitive 3) of Arabidopsis thaliana encodes a calcium-binding protein that is able to sense the cytosolic calcium signal elicited by salt stress. SOS3 activates the SOS2 protein kinase, which activates various ion transporters. SOS3 was cloned into a plasmid and expressed in Escherichia coli, allowing purification of the protein to homogeneity. Two crystals with different additive contents were grown. Both diffract to 3.2 A resolution and belong to space group I4(1), with unit-cell parameters a = 93.65, c = 80.08 A and a = 91.79, c = 85.78 A, respectively. A promising molecular-replacement solution has been found using neuronal calcium-sensor 1 as the search model. Interestingly, no solution was found using AtCBL2 (A. thaliana calcineurin B-like protein) structure as a search model, although this protein belongs to the same family and displays 50% sequence identity.