We develop a model of firms' auditor choices when presented with a heterogeneous group of investors. We show that firms' auditor choices in equilibrium depend on the composition of investors in the market. The signaling effect of choosing a high-quality auditor exists only when there is at least a certain proportion of sophisticated investors. If there is a sufficiently high proportion of sophisticated investors, then all firms will choose high-quality auditors. We also show that the overall audit quality in the market increases with an increasing proportion of sophisticated investors. When the audit market is differentiated and investors are heterogeneous, an increase in the penalty for firms that receive a qualified opinion will lead to a decrease in the overall audit quality in the market. Our conclusions remain valid even after taking audit fees, auditor quality change, and firm heterogeneity into consideration.