The majority of published human mitochondrial DNA sequence data are confined to hypervariable region I in the control region. By contrast, this paper focuses on a nucleotide site in hypervariable region II. Unlike most non-European populations whose mtDNA sequences have been studied in the literature, the British 'white Caucasian' population has a high level of variation at site 73 (following the site numbering by Anderson et al. 1981). This variation appears to have its origin largely in a mutation from guanine to adenine at that site with an estimated minimum age between 15,000 and 25,000 years. The data of Piercy et al. (1993) suggest that roughly half of the British 'white Caucasian' mitochondrial gene pool is descended from a common maternal ancestor who carried this mutation at site 73. This site also plays a central role in distinguishing the five major European mtDNA clusters identified in Richards et al. (1996). We suggest that the lineages carrying an A at site 73, together with some other lineages, may have their origins in a small founder population which expanded after the last glacial maximum about 20,000 years ago. We conclude that, in addition to region I sequences, site 73 is worth determining in studies of Caucasian populations.