Glycolysis occurs in almost every living cell as part of the energy metabolism. It forms a complex dynamical system, and might thus be capable of exhibiting complex phenomena. Simple oscillations have been observed frequently in suspensions of intact cells and in cell extracts, but only as transients. We have obtained sustained simple and complex oscillations in glycolysis of cell-free yeast extract in a flow-reactor. Sustained oscillations enable a powerful, proven method of dynamical system theory to unravel the kinetics and make it possible to observe chaos. Chaos was predicted from models long ago but has not previously been observed experimentally. We report the first experimental observation of unforced chaotic oscillations in glycolysis. Copyright 1997 Academic Press Limited