SINCE the original work by Lush, Lamoreux and Hazel (1948) and by Robertson and Lerner (1949), there have been few attempts to estimate the heritability of viability amongst laying hens. It is, however, only through the accumulation of estimates that generalisations can be made regarding heritability of particular characters. Hence the records of a White Wyandotte flock, kept by the Agricultural Research Institute of Northern Ireland at Hillsborough, have been analysed for two years during which there was very high mortality, to estimate heritabilities of viability and of resistance to leucosis.The foundation stock for this flock was bought in 1950 from a number of farms on which there were no known cases of leucosis. Subsequently the flock was closed and all practicable steps were taken to avoid leucosis, by brooding the chicks and rearing the young stock at a distance from adult birds. As a requirement of the breeding …