Sardinian delta beta 0-thalassemia is an inherited syndrome characterized by the inactivity of the beta-globin gene and the persistent activity of the fetal gamma-globin genes, particularly the A gamma-globin gene. Previous mapping studies with restriction enzymes failed to show any abnormality in the non-alpha globin gene cluster. We have now examined the possibility that this syndrome might result from a single rather than two different defects. Restriction enzyme polymorphisms linked to the delta beta 0-thalassemic non-alpha globin fragments were defined providing the basis for cloning the delta beta 0-thalassemic beta-globin gene from the DNA of a heterozygous patient. This gene appears to carry a C----T single mutation causing the appearance of a stop codon at amino acid position 39 of the beta-globin gene. This mutation was previously reported in beta 0-thalassemic patients, in linkage with different haplotypes. We conclude that Sardinian delta beta 0-thalassemia is the result of two separate mutations, the former one (unknown) responsible for persistent expression of gamma-globin genes, the latter for beta 0-thalassemia.