Bacteriophage Mu C protein is an activator of the four Mu late promoters that drive the expression of genes encoding DNA-modification as well as phage head and tail morphogenesis proteins. This report describes the purification and cocrystallization of wild-type and selenomethionine-substituted C protein with a synthetic late promoter P(sym), together with preliminary X-ray diffraction data analysis using SAD phasing. The selenomethionine peak data set was collected from a single crystal which diffracted to 3.1 A resolution and belonged to space group P4(1) or P4(3), with unit-cell parameters a = 68.9, c = 187.6 A and two complexes per asymmetric unit. The structure will reveal the amino acid-DNA interactions and any conformational changes associated with DNA binding.