During 1991–1993 and 1998–1999, a leafblotch miner, Micrurapteryx salicifoliella (Chambers), infested willows (Salix spp.) throughout a vast area in drainages of the Yukon and Kuskokwim Rivers, AK. The insect’s biology had not been studied and it was unknown from Alaska. Eggs were laid singly, cemented to the epidermis of undersides of leaves. Hatched larvae mined directly into leaves beneath the chorion. Five instars occurred. Mining by the first three instars created little external evidence of their presence. Fourth and fifth instars, however, created conspicuous necrotic, reddish, blotches that often covered the upper leaf surface of susceptible host willows. Mature larvae exited through slits made on the undersides of leaves and spun cocoons on either leaf surface before pupating from mid-July into August. Adults emerged in late July and August and overwintered in that stage. Ten species of willows were infested, severity of which differed somewhat between localities and species. Feltleaf willow, S. alaxensis (Andersson) Coville, was not infested, apparently due to its under leaf surface being covered by a protective felt-like mat of hairs that prevented attachment of eggs to the epidermis, a condition deemed to be critical to survival of larvae upon hatching.