Single crystals of a low molecular weight phosphotyrosine protein phosphatase from bovine liver have been grown. The crystals belong to space group P2(1)2(1)2(1), have cell dimensions a = 46.3 A, b = 62.2 A, c = 62.7 A and diffract to better than 2.0 A resolution. The crystals are well suited for high resolution X-ray studies.