Frequent recombination occurs during replication in all retroviruses examined. This increases the genetic variation in the retroviral population and may be of importance in the evolution of the virus. Maedi-visna virus (MVV), a retrovirus of sheep, has a highly variable envelope gene. In a previous experiment, 20 sheep were infected with an uncloned strain of MVV and virus was isolated at regular intervals for 7 years. We sequenced the envelope genes of a number of these strains and found evidence for recombination that may have contributed to the observed high frequency of antigenic variants.