This paper is motivated by the debate which currently exists concerning the joint effect on the UK audit market of audit firm mergers and increasing competitive pressures. Based on a large sample of 2,079 domestic listed companies, measures of seller concentration are calculated for the period 1987 to 1991. The three principal factors which have contributed towards the change in concentration are identified as audit firm mergers, voluntary realignments (auditor switching) and the auditor distribution of newly listed companies. The relative impact of these factors on seller concentration is measured. The results of this study are placed in perspective via cross-study longitudinal and inter-country comparisons of concentration ratios and annualized rates of auditor change. Results indicate an increasing level of seller concentration in the UK market for audit services, which is also high relative to other countries, and a high and rising instability in client-auditor relationships. Policy implications are discussed.