The purpose of this study is to gain insights into the determinants of audit fees in a not yet analyzed, special audit market, namely the highly culturally differentiated and supplier-concentrated audit market in Switzerland. Using a sample of listed firms' panel data, we first investigate whether audit firms compete on regional markets rather than on the national market. Furthermore, we examine the relationship between gained audit fees and auditor size - a proxy for market dominance and audit quality - to obtain a deeper understanding of the markets' structures. We identify two regional audit markets: the French-speaking western and the German-speaking eastern part of Switzerland. We provide evidence of KPMG's audit fee premium in the nation-wide as well as the regional audit markets. In addition, we detect evidence of an audit fee discount by Deloitte in the western audit market