This study utilizes a dominant-bank model to investigate whether an increase in retail loan and deposit-market concentration increases the incentives for both dominant and fringe banks to monitor their loans and thereby improve the quality of their loan portfolios. It shows that the effects on banks' incentives to engage in monitoring aimed at eliminating loan default losses in response to increased concentration of retail market shares of loans and deposits depend critically on whether the banks' asset and liability choices are interdependent. When the asset and liability decisions of both dominant and fringe banks are independent, a shift in market shares in favor of the dominant bank generates a straightforward increase in the incentives of all banks to monitor their loans. Under portfolio interdependence, the effects on monitoring outcomes at dominant banks and at banks within the competitive fringe depend on more complicated configurations of parameters. This fact helps explain mixed empirical evidence on the relationships between bank competition and measures of bank risk and soundness.