Cretaceous/Tertiary boundary sediments are now widely recognized to contain the record of a large asteroid or comet impact event(1), probably at the site of the Chicxulub crater on the Yucatan peninsula(2). After nearly two decades of intensive research, however, much remains unknown about the specific nature of the projectile and of the impact event itself. Here we describe a 2.5-mm fossil meteorite found in sediments retrieved from the Cretaceous/Tertiary boundary in the North Pacific Ocean that we infer may be a piece of the projectile responsible for the Chicxulub crater. Geochemical and petrographic analyses of this meteorite indicate that it probably came from a typical metal- and sulphide-rich carbonacous chondrite rather than the porous aggregate type of interplanetary dust considered typical of cometary materials(3). The fact that meteorite survival should be enhanced by impacts at low (asteroidal) velocities(4) also implies that this meteorite had an asteroidal rather than a cometary origin.