Bacteriophage varphi12 protein P7 is a structural component of the polymerase complex and ensures stable packaging of the genomic RNA. varphi12 P7 has been cloned, purified and crystallized. Crystals belong to space group P3(2)21, with unit-cell parameters a = 75.7, b = 75.7, c = 45.2 A, alpha = 90, beta = 90, gamma = 120 degrees , and diffract beyond 2.0 A. Multiple anomalous dispersion data have been collected from crystals of selenomethionylated P7. Mass spectroscopy showed proteolysis of the crystallized protein and a truncated form, P7DeltaC, gave crystals of similar morphology. Cross-linking experiments implicated the N-terminal domain of P7 as being essential for dimerization.