Abyssal peridotites and mid-ocean-ridge basalts (MORBs) are complementary products of the mantle melting and melt-extraction processes that create the ocean crust Studies of abyssal peridotites(1-4) and MORBs(2) have showed that the extent of mantle melting is high beneath hotspot-influenced shallow ridges, and is low beneath deep ridges away from hotspots. These results have led to the recognition of a global correlation of MORB composition with ridge depth(5), and to the notion that mantle temperature variation exerts the primary control on the extent of melting beneath ocean ridges(2,5-8). This conclusion is, however, based largely on data from slow-spreading ridges in the Atlantic and Indian oceans. At the fast-spreading East Pacific Rise (EPR), there is little correlation between MORB chemistry and ridge depth(9,10), an observation that has proved puzzling(8-12), Here we show that abyssal peridotites from the EPR(13-21) are extremely depleted in basaltic major-element components-significantly more so than peridotites from ridges away from hotspots in the Atlantic and Indian oceans-indicating that the EPR peridotites are residues of the highest extents of melting. These abyssal peridotite data and existing MORB major-element data(12) both suggest that the extent of mantle melting beneath normal ocean ridges increases with increasing spreading rate.