We describe a novel mutation in the delta globin gene of a compound heterozygote for delta o thalassemia and a deletion type G gamma + (A gamma delta beta) zero thalassemia. The delta was amplified using the polymerase chain reaction (PCR), and the amplified material was used in a direct sequencing experiment. The nucleotide sequence of the mutant delta gene showed that the insertion of an extra nucleotide at the third position of codon 91 in the second exon, which gives rise to a premature stop codon at position 94, leads to the silencing of this gene. The presence of the mutation in the carriers of delta-thalassemia in this family was confirmed by dot blot hybridization. A possible model for the insertion of the extra nucleotide is discussed.