Human coronavirus HKU1 (HCoV-HKU1) belongs to coronavirus group II and encodes 16 nonstructural proteins (nsps) which mediate genome replication and transcription. Among these nsps, nsp9 has been shown to possess single-stranded DNA/RNA-binding properties. The gene that encodes HCoV-HKU1 nsp9 was cloned and expressed in Escherichia coli and the protein was subjected to crystallization trials. The crystals diffracted to 2.7 A resolution and belonged to space group P2(1)2(1)2, with unit-cell parameters a = 83.5, b = 88.4, c = 31.2 A, alpha = beta = gamma = 90 degrees and two molecules per asymmetric unit.