HK97 is a member of the Siphovirus family of dsDNA bacteriophages. It is similar in architecture to bacteriophage lambda, the type member of this family, with an icosahedral capsid of triangulation number T = 7. No high-resolution structural information is available for the dsDNA phages, and HK97 is the only dsDNA bacteriophage capsid to produce crystals which diffract X-rays. At 650 A in diameter, the large size of the particle and resultant large unit cell create crystallographic challenges. The empty Head II (mature) particles were expressed in Escherichia coli and assembled in vitro, but they have the same morphology as the mature HK97 capsid. Previously reported Head II crystals diffracting to 3.5 A resolution are examined here in detail. Although the cell dimensions suggest an orthorhombic lattice, further analysis demonstrated that the space group was monoclinic. This has been confirmed by the present study. Images were recorded on the F1 beamline at CHESS and they were processed and scaled, resulting in a data set with a cumulative completeness of 65% and a scaling R factor of 7.7% to 7 A. The cell dimensions after post-refinement were a = 580, b = 626, c = 788 A, beta = 90.0 degrees. From the particle dimensions determined by cryo-electron microscopy (cryo-EM), there were determined to be two particles per unit cell. Systematic absences of even reflections along the 0k0 lattice line indicate that the space group is P21. The rotation function was used to determine the orientation of the particles in the unit cell and to confirm the space group. An icosahedral twofold axis is approximately, but not exactly, aligned with the crystallographic screw (b) axis. An icosahedral twofold axis orthogonal to the one approximately parallel to the b axis, is rotated 18 degrees away from the a axis. The centers of the two particles must be positioned close to the minimum-energy packing arrangement for spheres, which places one particle at ((1/4), 0, (1/4)) and the other particle at ((3/4), (1/2), (3/4)). The particle position and orientation were confirmed by calculating a Patterson function. The particles interact closely along icosahedral threefold axes, which occurs both along the crystallographic a axis and along the b axis. The particle dimensions derived from this packing arrangement agree well with those determined by cryo-EM and image reconstruction. The cryo-EM reconstruction will be used as a model to initiate phase determination; structure determination at 7 A is under way.