The top 10 largest audit firms performed 60.7 percent of all public company audits in the U.S. in 2015, a nearly 4 percent increase over the year before, according to Audit Analytics. While the top 10 dominated all other audit firms, the top 6 within this list dominated the rest: the Big Four firms, plus BDO and Grant Thornton, accounted for 96.8 percent of audits of large accelerated filers, 71.9 percent of accelerated filers, and 66 percent of non-accelerated filers. The exception was smaller filers, where the remaining firms accounted for 76.8 percent of audits. 

The top Big Four firm, in terms of number of public company audits performed, was Ernst and Young, which alone accounted for 14.3 percent of them, followed by PwC at 10.3 percent, KPMG at 10 percent, Deloitte and Touche at 9.6 percent.