The amino-terminal 321 residues encoding the catalytic domain of human protein tyrosine phosphatase 1B (molecular mass 37 kDa) has been expressed in Escherichia coli, purified to homogeneity and crystallized. The crystals diffract to 2.4 A resolution when exposed to synchrotron radiation and belong to space group P3(1)21 (or its enantiomorph P3(2)21) with a = 88.4 A, b = 88.4 A, c = 104.0 A, alpha = beta = 90.0 degrees, gamma = 120.0 degrees. There is one molecule of protein tyrosine phosphatase 1B per asymmetric unit and the crystal form is suitable for the determination of the atomic structure of the enzyme.