Despite the worldwide public health impact of malaria, neither the mechanism by which the Plasmodium parasite detoxifies and sequesters haem, nor the action of current antimalarial drugs is well understood. The haem groups released from the digestion of the haemoglobin of infected red blood cells are aggregated into an insoluble material called haemozoin or malaria pigment. Synthetic beta-haematin (Fe-III-protoporphyrin-IX)(2) is chemically(1,2). spectroscopically(2,3) and crystallographically(4) identical to haemozoin and is believed to consist of strands of Fe-III-porphyrin units, linked into a polymer by propionate oxygen-iron bonds. Here we report the crystal structure of beta-haematin determined using simulated annealing techniques to analyse powder diffraction data obtained with synchrotron radiation. The molecules are linked into dimers through reciprocal iron-carboxylate bonds to one of the propionic side chains of each porphyrin, and the dimers form chains linked by hydrogen bonds in the crystal. This result has implications for understanding the action of current antimalarial drugs and possibly for the design of new therapeutic agents.