This paper explores the structure of the Canadian audit market between 1901 and 1941 based on a sample of 3661 financial statements from 956 firms. Two aspects of the market are examined: first, the overall degree of market concentration, and second, the existence of market segmentation. In addition, a specific concern of the paper is to analyse competition between domestic accounting firms and the international accounting firms leading to the merger of major independent Canadian firms with international accounting firm networks after World War Two. The data show a pattern of increasing concentration during the period among a small set of domestic and international firms. The data identify both a national market and a series of regional markets for audit services. There is also evidence of market segmentation by industry and stock exchange listing. Overall, the evidence suggests that the early Canadian audit market was competitive but fragmented into a series of niche markets. Domestic firms were able to compete with the international firms but the market was becoming increasingly concentrated.