The problem of the research conducted in the thesis is to determine the level of concentration in the market of audit services after the entry into force of the new Audit Act, which was introduced after the entry into force of Directive 2014/56 / EU. The aim of the research is to determine the degree of concentration (using different concentration measures, ie Gini coefficient of concentration, concentration ratio and HHI index) in the audit services market in the Republic of Croatia in relation to the number of clients, total assets and revenues. In addition, the aim of the research to be conducted is to determine whether the entry into force of the new Audit Act has led to changes in the audit services market, and thus to changes in the degree of concentration in the audit services market. One of the goals is to investigate whether there is a significant difference in the number of clients, total assets and revenues, when it comes to audit firms that are part of the international network of audit firms, since it is assumed that companies that are part of the international network have greater capacity. The empirical part of the paper was compiled on the basis of data from the financial statements of 106 audited listed companies. Considering the calculated concentration measures, hypothesis H1 is rejected: The degree of concentration on the market of audit services in the Republic of Croatia measured by the number of clients is low because it can be concluded that the concentration on the market is moderate. Measured by total assets and revenues of clients, the market concentration is high, and therefore hypothesis H2 is accepted: Measured by revenues and assets of the audited entity, the market of audit services in the Republic of Croatia is moderately to highly concentrated. It can be concluded that although the concentration in the market according to the number of clients is moderate, if the revenues and total assets of clients are analyzed, the market of audit services is highly concentrated. It is possible to conclude that in the period from 2016 to 2019 there were no significant changes in the level of concentration in the audit services market, but that there were changes when it comes to the leading audit firms in terms of number of clients, ie PWC d.o.o. partly loses the status of one of the leading audit firms.