Development of the chicken retina was investigated through clonal analysis using retroviral vectors. Replication-incompetent retroviral vectors encoding either human placental alkaline phosphatase, beta-galactosidase, or a viral core protein were used in paired combinations to infect retinal progenitor cells from Embryonic Days 2.5-7.0. Labeled clones were analyzed late in embryonic development after retinal histogenesis was complete. The early accessibility of the chicken retina, combined with its large final size, resulted in the labeling of much larger clones than had been reported previously in other species. The clones were composed of many cell types, supporting previous conclusions from other vertebrates that the progenitor cells of developing retina are multipotent. The majority of clones derived from early infections consisted of multiple tightly clustered arrays of cells accompanied by dispersed individual cells. Clonal complexity and tangential dispersion were greater in peripheral than central retina and decreased considerably with increasing age of infection. These observations suggest that early during retinal development, shortly after optic cup formation, there is considerable mixing of progenitor cells.