Bacteriophage phi812 is a lytic phage from the family Myoviridae. Phi812 infects most staphylococcal strains including those resistant to antibiotics [1]. The structure of this bacteriophage was previously determined by cryo electron microscopy [2] but functions of individual proteins remain unclear. Here we present our work towards resolving the structure of protein gp99 that forms the neck region of phi812. The protein may play a role in a regulation of the genome release from the virion during infection. Gp99 was cloned and expressed in E. coli and purified by affinity and size exclusion chromatography. A suitable crystallization condition was found and X-ray diffraction data with a resolution of 2.3 A were collected. To solve the phase problem, a variety of methods had been tried, including molecular replacement, heavy atom soaking and co-crystallization, and seleno-methionine incorporation. The structure of gp99 will help explain the mechanism of bacterial infection by bacteriophage. As there is a rising number of antibiotic resistant bacterial strains causing severe illnesses, phage therapy has a high potential. For this purpose, detailed knowledge of bacteria-phage interactions is vital.