SPATIAL and temporal order often arises in homogeneous systems driven away from equilibrium(1). Sufficiently far from equilibrium, however, highly ordered behaviour typically degenerates into spatiotemporal chaos. The mechanisms underlying this qualitative transition can be clarified by studying a model system comprising a continuous field of coupled nonlinear oscillators. Analyses of the nonlinear partial differential equation that describes this model elucidate generic dynamical features that characterize spatially extended nonequilibrium systems; moreover, they predict an archetypal pathway(2,3) for the development of spatiotemporal chaos through the spontaneous generation of topological singularities, or ’defects’. The resulting highly irregular state, known as ’defect-mediated turbulence’, has not, however, been observed previously in a real system. Here we report an experimental observation of the transition from ordered behaviour to defect-mediated turbulence in the pattern-forming chemical system known as the Belousov-Zhabotinsky reaction. A regular spiral pattern dominates in the ordered state, but as the system is driven further from equilibrium, the spiral becomes unstable and generates hundreds of defects. This observation substantiates a basic mechanism which should underlie the transition to spatiotemporal chaos in a wide variety of pattern-forming systems.