B(o) -Thalassemia is a genetic disorder causing a complete absence of B-globin chain in hemoglobin. In Thai population, B...-thalassemia associated with HbE (B(26)GAG--->AAG) is found in high frequency. More than 50 types of molecular mutations in B-thalassemic gene have been reported, including large sequence deletion, short sequence deletion or insertion, and single base substitution. The presence of large sequence deletion (619 b and 3.4 kb) were screened in 60 Thai patients with B...-thalassemia/HbE using fragment length analysis, but none were detected. Single base substitution or insertion and short sequence deletion in B-globin gene of 30 Thai patients with B...-thalassemia/HbE were screened by direct gel hybridization of BamH I fragmented DNA and by dot-blot hybridization of amplified DNA with a set of allele specific oligodeoxyribonucleotide probes. Frequency of mutations detected were as follows : 17 cases of a 4 base-pair deletion at codons 41-42, 5 cases of amber mutation (TAG) at codon 17, one case each of a single base substitution (G-->C) at position 5 of IVS-I, a single base substitution (C-->T) at position 654 of IVS-II, and ochre mutation (TAA) at codon 35. However, no mutations of single nucleotide (A) insertion at codons 71-72 were found. These data provide a basis for future application of prenatal diagnosis by DNA hybridization.