beta thalassemia is one of the most common genetic diseases worldwide resulting from aberrant beta-globin chain production. It is highly prevalent in regions with endemic malaria, but it is also present at low frequency in the indigenous populations of non-tropical areas such as Britain. Screening beta thalassemia trait individuals from Northern Ireland has detected 2 Mediterranean mutations, 39 (C --> T) and IVS-I-110 (G --> A); the previously reported IVS-II-850 (G --> A) mutation originally described in individuals of Scottish/English ancestry; and 2 novel mutations, initiation codon A --> C and 109 delG. Haplotype analysis indicates that the Mediterranean mutations are present on previously described haplotypes, suggesting that they have arisen due to migration. It remains to be established whether the novel mutations have arisen de novo in Northern Ireland.