This paper aims at investigating the effects of concentration on competition and cost efficiency in the French audit market. Competition is measured with the Rosse-Panzar model, while cost efficiency is estimated with the stochastic frontier approach. Cost efficiency levels are estimated at around 75%, while the nature of competition appears to be monopolistic. Dynamic analysis shows a reduction in competition, and a decrease in cost efficiency for Big-Four and non-Big-Four firms between 1999 and 2003. We therefore provide support for a negative impact of concentration on competition and cost efficiency.