ABSTRACT: A report issued by the U.S. General Accounting Office (GAO) in 2003 identified auditors’ industry expertise as a critical factor for firms choosing an auditor, and highlighted the extreme levels of auditor concentration in some industries. We posit that the investment opportunity set (IOS) plays a fundamental role in determining whether an industry is an attractive target for auditor specialization. When industry-specific IOS is high, specialist auditors make costly investments in industry-specific knowledge, allowing them to offer a differentiated product and to create entry barriers for other audit firms. When the IOS of firms within an industry is relatively homogeneous, auditors can transfer such knowledge across clients in the industry more easily, resulting in cost savings and scale economies. However, greater homogeneity of IOS in an industry can also increase a client’s aversion to sharing an auditor with its competitors because of concerns about transfers of proprietary information, suggesting that industries with relatively homogeneous IOS are less likely to be dominated by a single auditor. We show that auditor concentration in an industry relates positively to both the level and homogeneity of IOS in the industry, while auditor dominance relates negatively to industry IOS homogeneity. Further, we find that audit fees are positively associated with both levels and homogeneity of industry IOS.