The distribution of 12 endogenous retrovirus loci from 13 White Leghorn flocks of diverse geographical origin was examined, using a total of 508 birds. Only one locus, designated ev-1, was observed to be almost universally present (506 of 508 birds) and thus predominantly homozygous in this breed of chicken. By contrast, 11 other viral loci were observed at much lower frequencies, 3 to 30%, and were occasionally restricted to individual flocks. To further characterize the commonly occurring ev-1 viral locus and to provide a foundation for determining the mechanism by which these 12 endogenous retrovirus loci were generated, a new technique of indirect in situ hybridization was used to localize the chromosomal position of ev-1. By using a combination of specific probes, the position of ev-1 was localized to a small region on the long arm of chromosome 1. This research represents oen of the first instances in which a defined segment of nonreiterated DNA coding for a known set of genes has been localized to a specific region on a vertebrate chromosome by in situ hybridization.