THE Chengjiang fossil Lagerstätte is one of the earliest and most important palaeontological sites from the Phanerozoic era1,2, about 530 million years ago3. It yields extremely abundant and remarkably preserved soft-bodied fossils and shells with soft parts of various kinds, including bradoriids4-6, trilobites7,8, crustaceans9, brachiopods, worms, sponges, algae and many unknown forms10-13. One of these fossils is Yunnanozoon14, which we reinterpret here as the earliest known hemichordate. Possessing half of the characteristic chordate features and providing an anatomical link between invertebrates and chordates15, Hemichor-data is a minor but important phylum in evolutionary biology. Hemichordates comprise two main groups: the enteropneusts, or 'acorn worms', and the pterobranchs. Apart from the presumable inclusion of graptolites in pterobranchs16-19, there are very few hemichordate fossils2,17,20. Although Yunnanozoon is superficially similar to the chordates21, its typical tripartite body plan is broadly consistent with that of living balanoglossid hemichor-dates (enteropneusts).