We have investigated the molecular basis of α thalassaemia in the so-called ‘Cape Coloured’population of Cape Town. DNA from 17 cases was analysed by Southern blotting and hybridization with an α globin complementary DNA probe. Three types of α thalassaemia genetic determinants were detected: the 3·5 kb deletion which inactivates one α globin gene per chromosome (-α/ haplotype), a much larger deletion which removes both α globin genes (––/ haplotype), and a non-deletion determinant which leaves both α globin genes intact. The interaction of these determinants with each other or with the normal chromosome (αα/) produced the phenotypes α thalassaemia silent carrier, α thalassaemia trait and Hb H disease. All cases of the -α/ haplotype result from the rightward deletion which removes the Bgl II site between the duplicated α globin genes. The predominance of the -α/ haplotype (21 out of the 28 α thalassaemia determinants) over the ––/ haplotype is consistent with the low incidence of Hb H disease and the apparent absence of Hb Bart's hydrops fetalis in this population group.