Our theoretical calculations indicate that the massive pulsating DA white dwarf, BPM 37093, has a crystallized ion interior. Our estimates for the crystallized mass fraction depend on our assumption for the interior composition and range from 10% for pure carbon and low total mass to more than 90% for pure oxygen and high total mass; the mass of this white dwarf suggests on theoretical grounds that the core composition is unlikely to be lighter than oxygen. Our best estimate is that the crystallized mass fraction exceeds 90%. BPM 37093 is the first known example of a pulsating white dwarf star with a potentially crystalline interior. We point out that future observations of the pulsation properties of this object are essential for carrying out asteroseismological measurements of the crystallized mass fraction of its interior. Uncertainties in the nature and extent of crystallization and its associated effects are the largest source of uncertainty in calculating the ages of the coolest white dwarf stars—an important chronometer for the Galactic disk. In addition, crystallization plays an important role in our understanding of neutron star crusts. Investigations of the pulsation properties of this star promise to provide us with the first empirical tests of the theory of crystallization in stellar interiors, now more than 35 yr old
