Apoptosis is stimulated by the insertion of Bax from the cytosol into mitochondrial membranes. The solution structure of Bax, including the putative transmembrane domain at the C terminus, was determined in order to understand the regulation of its subcellular location. Bax consists of 9 α helices where the assembly of helices α1 through α8 resembles that of the apoptosis inhibitor, Bcl-x_L. The C-terminal α9 helix occupies the hydrophobic pocket proposed previously to mediate heterodimer formation and bioactivity of opposing members of the Bcl-2 family. The Bax structure shows that the orientation of helix α9 provides simultaneous control over its mitochondrial targeting and dimer formation.