The severe nuclear accident at Chernobyl in 1986 resulted in the worst reported accidental exposure of radioactive material to free-living organisms(1). Short-term effects on human populations inhabiting polluted areas include increased incidence of thyroid cancer(2), infant leukaemia(3), and congenital malformations in newborns(4). Two recent studies(5,6) have reported, although with some controversy(7,8), that germline mutation rates were increased in humans and voles living close to Chernobyl, but little is known about the viability of the organisms affected(9). Here we report an increased frequency of partial albinism, a morphological aberration associated with a loss of fitness, among barn swallows, Hirundo rustica, breeding close to Chernobyl. Heritability estimates indicate that mutations causing albinism were at least partly of germline origin. Furthermore, evidence for an increased germline mutation rate was obtained from segregation analysis at two hypervariable microsatellite loci, indicating that mutation events in barn swallows from Chernobyl were two-to tenfold higher than in birds from control areas in Ukraine and Italy.