This paper examines the issue of auditor concentration in the UK during the period from 1991 to 1995. It shows that in 1995 the Big Six held 75% of the total number of audits and collectively earned 92% of the total audit fees. There was only a small increase in auditor concentration during the five year period, resulting from companies switching from small audit firms to Big Six and newly listed companies choosing a Big Six firm. The paper also examines auditor concentration within industries. Finally, the study assesses the measurement methods most commonly used in auditor concentration studies.