In the present paper, abnormal feather development associated with an in ovo avian leukosis virus (ALV) subgroup J infection in specific pathogen free broilers is reported. A description of gross lesions is given, together with the results of light microscopic, immunohistochemical and ultrastructural studies to further elucidate the characteristics of the feather pathology and to assess the presence of ALV subtype J in broiler feathers. The feather lesions found, which were mainly restricted to primary and secondary remiges, were thinness and increased transparency of the calamus and sparseness of the vane. In a few cases, feather growth retardation was present, while the vane remained ensheathed. Light microscopy did not reveal abnormalities, although immunohistochemically prominent anti-p27 staining was found in the central area of the feather pulp between cells forming the barbular epithelium. Transmission electron microscopy showed the presence of retroviral particles in various structures of the feather (epidermal collar and intermediate layer of feather epidermis), most virus being present in the same area that stained most prominently by immunohistochemistry and where apparently intra-epithelial gaps were seen. ALV was isolated from the upper two-thirds of primary remiges of almost all inoculated birds and 40% of contact birds. Polymerase chain reaction of cell cultures showed that the virus isolates belonged to subgroup J. This study shows that subtype J virus can be massively present in feather tissue of infected birds, which could have consequences for its transmission and diagnosis.