Mid-oceanic-ridge basalts (MORB) form by partial melting of material in the convecting upper mantle. The range in isotopic compositions observed in MORB is inconsistent with the ultradepleted isotopic compositions observed in many abyssal peridotites. These results have called into question the prevailing hypothesis that abyssal peridotites (APs) are simple residues of recent MORB melting, which should result in the two reservoirs having the same range in isotopic compositions. We examined xenoliths that, based on their chemical features (e.g., light rare earth element depleted, fertile major element compositions, Sr-Nd-Pb isotopes similar to estimates for depleted MORB mantle), are interpreted to be derived from the convecting upper mantle, in order to evaluate the potential for isotopically ultradepleted domains to contribute significantly to MORB petrogenesis. Our data support the idea that isotopically ultradepleted peridotite is widely distributed in the upper mantle, and we demonstrate that ultradepleted domains are capable of contributing to MORB petrogenesis. An isotopically enriched component, such as recycled oceanic crust, in the MORB source mantle can account for the lack of MORB with ultradepleted isotopic compositions.