This is the publisher's version, also available electronically from http://scripts.iucr.org/cgi-bin/paper?S0907444999010318.Guanidinoacetate methyltransferase is the enzyme which catalyzes the last step of creatine biosynthesis. The enzyme is found ubiquitously and in abundance in the livers of all vertebrates. Recombinant rat-liver guanidinoacetate methyltransferase has been crystallized with guanidinoacetate and S-adenosylhomocysteine. The crystals belong to the monoclinic space group P21, with unit-cell parameters a = 54.8, b = 162.5, c = 56.1 Å, [beta] = 96.8 (1)° at 93 K, and typically diffract beyond 2.8 Å