Equine arteritis virus (EAV) is an enveloped positive-sense RNA virus belonging to the Arteriviridae family, which also includes the porcine pathogen PRRSV and is genetically and structurally related to the coronaviruses. EAV is an important equine pathogen that has caused significant economic losses to the horse-breeding industry and has been difficult to control. The EAV virion consists of a genome-containing nucleocapsid core made of nucleocapsid (N) protein surrounded by a lipid envelope containing several membrane proteins. Here, the crystal structure of the capsid-forming domain of the EAV N protein is presented at 2.0 Å resolution. The dimeric N-protein structure is similar to the previously determined structure of the N protein from PRRSV, with most differences localized to the terminal helices and flexible loops. The N protein is organized as dimers of dimers in the crystal, which may reflect the arrangement of the protein in the viral nucleocapsid. © International Union of Crystallography 2007.