Bacteriophage Mu baseplate protein gene product 44 (gp44) is an essential protein required for the assembly of viable phages. To investigate the roles of gp44 in baseplate assembly and infection, gp44 was crystallized at pH 6.0 in the presence of 20% 2-methyl-2,4-pentanediol. The crystals belong to space group R3, with unit-cell parameters a = b = 127.47, c = 63.97 A. The crystals diffract X-rays to at least 2.1 A resolution and are stable in the X-ray beam and are therefore appropriate for structure determination. Native data have been collected to 2.1 A resolution using a DIP6040 image-plate system at beamline BL44XU at the SPring-8 facility in Japan.