Choroid plexus (GCP-3) cell cultures were prepared from an adult goat with symptoms of visna. The GCP-3 cell layer had partly fused into large multinucleated giant cells and electronmicrographs showed virus particles morphologically indistinguishable from sheep visna virus (SVV). A virus, designated goat visna virus (GVV), was subsequently purified from the GCP-3 cultures. The virus particles have a density of 1.15 g/ml and a high molecular weight RNA similar in size to that of SVV. A virion-associated DNA polymerase was identified which is stimulated to the same extent as the SVV polymerase by different synthetic RNA and DNA template-primer combinations and which shows the same Mg2+ and Mn2+ stimulation optima. Polypeptide analysis by SDS-PAGE revealed that the virion proteins of GVV and SVV had similar molecular weights. By immunodiffusion tests it was demonstrated that the major internal proteins of GVV and SVV are related. Consequently, we conclude that GVV should be classified as a retrovirus and that it is closely related to visna virus of sheep.