STARTING with the discovery of an iridium anomaly at the Cretaceous/Tertiary boundary in Italy 1, the idea that a large asteroid or comet struck the Earth at the end of the Cretaceous period has gained wide acceptance 2-4 although some workers suggest that massive volcanic eruptions can also explain the observations 5. The abundance of small diamonds, 3-5 nm in size, in chondritic meteorites 6,7 prompted us to search for such diamonds in the Cretaceous/Tertiary boundary clay of the Red Deer Valley of Alberta (the 'Knudsen's farm' locality). Dissolution and oxidation of this clay yielded 45 ng g-1 of a white fraction, consisting of more than 97% carbon, which was absent from rocks above and below the boundary layer. We present evidence that this material is indeed diamond, which strengthens further the case for an extraterrestrial impact. The diamond/iridium ratio in the boundary clay may constrain the type of impactor; our estimate (1.22:1) is close to the value found in type C2 chondritic meteorites 2.