The putative receptor-binding domain (domain III) of the flavivirus Langat envelope glycoprotein has been crystallized using the hanging-drop vapor-diffusion method at 277 K. Two distinct crystal morphologies were observed to grow under the same conditions. The crystal forms both belong to a trigonal space group, P3(1)21 or P3(2)21, with unit-cell parameters a = 80.93, c = 132.1 A and a = 104.8, c = 219.5 A for forms I and II, respectively. Complete data sets to 2.9 and 3.35 A, respectively, have been collected at 100 K with Cu Kalpha X-rays from a rotating-anode generator.