We observed two White-tailed Ptarmigan (Lagopus leucurus) in white winter plumage (basic) in Colorado during late July through August during 35 years of field study. Both individuals were adult (>2 yr of age) females; one was collected and the second examined and photographed. Examination of the plumage revealed that neither had molted from winter plumage into their nuptial (alternate) or postnuptial (supplemental) plumage. We speculate that retention of the white winter plumage was the result of failure of the pituitary and thyroid glands to secrete sufficient hormones to initiate replacement of white body feathers with normal breeding plumage feathers. Perhaps this failure of the pituitary and thyroid glands was not age related but instead gender related. Despite conspicuous white plumage during summer, behavior and survival of both birds appeared normal.