The key response-regulator gene of light regulation, rcp1, from Synechocystis sp. has been overexpressed, purified and subsequently crystallized using ammonium sulfate as a precipitant in forms suitable for X-ray crystallographic studies. A native data set was collected to a resolution of 2.5 A at cryogenic temperature. The crystals belong to the hexagonal space group P6(3), with unit-cell parameters a = b = 89.04 (5), c = 60.29 (3) A. The Matthews parameter suggests that Rcp1 crystallizes with two molecules per asymmetric unit.