From September 2001 to February 2005, observations of an island population of the New Zealand stitchbird (Notiomystis cincta) revealed a progressive feather-losing dermatitis, which developed during the breeding season around the birds' eyes, base of the bill, and ventral neck. The lesions were significantly more likely to develop in males (96%) than females (51%), with males exhibiting a more severe form of the condition at the end of the breeding season. Histology from a dead bird revealed the presence of ovoid burrowing mites within the lesions, and isolation of mites from skin crusts of a live bird were identified as Knemidocoptes spp. Although other factors might be involved in the expression of the condition, Knemidocoptes appears to be a likely causative agent in the development of skin lesions in this population.