BPM 37093 is the only hydrogen-atmosphere white dwarf currently known which has sufficient mass (~1.1 M&sun;) to theoretically crystallize while still inside the ZZ Ceti instability strip (Teff˜12 000 K). As a consequence, this star represents our first opportunity to test crystallization theory directly. If the core is substantially crystallized, then the inner boundary for each pulsation mode will be located at the top of the solid core rather than at the center of the star, affecting mainly the average period spacing. This is distinct from the “mode trapping” caused by the stratified surface layers, which modifies the pulsation periods more selectively. In this paper we report on Whole Earth Telescope observations of BPM 37093 obtained in 1998 and 1999. Based on a simple analysis of the average period spacing we conclude that a large fraction of the total stellar mass is likely to be crystallized. Based on observations obtained at: Observatório do Pico dos Dias (OPD) Brazil, the European Southern Observatory (ESO) Chile, South African Astronomical Observatory (SAAO), Mt. John University Observatory (MJUO) New Zealand, Siding Spring Observatory (SSO) Australia, and Cerro Tololo Inter-American Observatory (CTIO), a division of the National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. under cooperative agreement with the National Science Foundation.