Bcl-2 family proteins are key regulators of apoptosis and have recently been shown to modulate autophagy. The tumor suppressor Beclin 1 has been proposed to coordinate both apoptosis and autophagy through direct interaction with anti-apoptotic family members Bcl-2 and/or Bcl-X(L). However, the molecular basis for this interaction remains enigmatic. Here we report that Beclin 1 contains a conserved BH3 domain, which is both necessary and sufficient for its interaction with Bcl-X(L). We also report the crystal structure of a Beclin BH3 peptide in complex with Bcl-X(L) at 2.5A resolution. Reminiscent of previously determined Bcl-X(L)-BH3 structures, the amphipathic BH3 helix of Beclin 1 bound to a conserved hydrophobic groove of Bcl-X(L). These results define Beclin 1 as a novel BH3-only protein, implying that Beclin 1 may have a direct role in initiating apoptotic signaling. We propose that this putative apoptotic function may be linked to the ability of Beclin 1 to suppress tumor formation in mammals.