SummaryHaemoglobin allele frequencies were examined in 195 ewes and 6 rams belonging to the Assaf cross (Awassi x East Friesian). The frequency of the Hb A allele was 0.14 indicating a decline in Hb A frequency in the Assaf during the last 20 years. Ewes belonging to the AB genotype showed a significant advantage over the BB genotype in both lamb and milk production in the first lactation. It is suggested that the haemoglobin polymorphism in the Assaf is maintained due to an advantage of AB heterozygotes for production and survival.