After the collapse of the auditing company Arthur Anderson, the auditing profession is gaining importance, especially in the protection of the public interest. Concerns about the dominant position of the four large audit firms are growing, as is the fear that the collapse of one of these firms could disrupt the entire audit and financial market. Previous research has shown that high concentration in the audit market services limits large companies in their choice of auditors, creating a temporary oligopoly that further hinders the entry of small and medium-sized companies into this market. This results in the need for legislative bodies to introduce legal regulations in the field of auditing. In the Republic of Croatia, auditing is regulated by the Auditing Act (OG 127/17) and Regulation (EU) no. 537/2014, therefore this research focuses on changes in the degree of concentration after the introduction of the Audit Act in 2017 (OG 127/17). This paper measured the concentration of the audit market in Croatia for 2020 and 2021, using standard measures such as the concentration ratio (CR4 and CR8), the Herfindahl-Hirschman index and the Gini coefficient of concentration. According to market shares based on the total assets and revenues of companies listed on the Zagreb Stock Exchange, the audit market in the Republic of Croatia is highly concentrated, with a growth trend between the years 2017 and 2021.