Six families with delta-thalassemia have been found in Japan, in which 10 individuals are delta-thalassemia homozygotes with complete deficiency of Hb A2 and 18 individuals are heterozygotes with low levels of Hb A2. In three families with 10 individuals among these, persistence of fetal hemoglobin of Swiss type was observed. Almost all the members of these families were free from clinical symptoms and hematological abnormalities of the red cells.