This study of companies listed on the stock exchanges in Tanzania, Kenya, Rwanda and Uganda has uncovered oligopolistic audit market structure. A total of 78 listed companies (74% of population) was analysed using audit fees’ data from 2013-2017. The study revealed that Big4 had a statistically positive influence on the audit fees paid by the listed companies. Non_Big4 as a bloc had a negative influence as some of the firms experienced reduction in audit fees after taking over from Big4. The Herfindahl-Hirschman Index (HHI) was 0.94 while Concentration Ratio (CR4 – clients) which measures client market concentration of the Big4 was 86% and CR4- audit fees was much higher at 96% with PwC and KPMG the top two. Only 5% of variation in audit fees could be attributed to variation in stock exchange. Audit fees increased by a compound annual growth rate of 8% but Non_Big4 remain marginalized and forced into fee discounting to retain the few clients.