The buoyant density of the Bucyrus strain of equine arteritis virus was studied by isodensity centrifugation on three types of gradients. The distribution patterns of infectivity were dependent on the type of gradient used. In sucrose gradients a single peak of infectivity at densities approximating 1.17 g/ml was obtained. From this gradient the total recovery of infectious virus varied between 80 and 90%. In cesium chloride gradients a rather broad band appeared in the form of a “saddled” peak with a range of densities from 1.180 to 1.215 g/ml, the total recovery being 90%. In potassium tartrate (KT) gradients two peaks were formed, one at 1.17 and another at 1.24 g/ml; the total recovery was 50%. Recentrifugation of fractions from KT gradient on sucrose gradients resulted in a changed distribution of infectivity for particles of higher density (1.24 g/ml).