Human rhinovirus 1A virions are found at two densities after cesium chloride gradient centrifugation. Dialysis of the virus against ammonium formate converts all the virus into the more dense form. Virus, dialyzed against ammonium formate and left for several days at 6 C forms crystals. Viral infectivity associated with washed crystals is lost by treatment at either pH 5 or pH 10.3, however, the integrity of the crystals is retained at the acidic pH. The crystals, as characterized by light and electron microscopy, are hexagonally prismatic with tapered, pyramidal ends. Electron images of replicated surfaces and thin sections of crystals show ordered packing of virus with a center-to-center repeat of 28.5 nm.