Using a unique dataset of non-listed firms that identifies the banks the firms transact with, we examine the effects of the largest-ever bank merger in Japan, that between Bank of Tokyo-Mitsubishi (BTM) and UFJ Bank (UFJ) in 2005. We focus on how the merger affected firms through their firm-bank relationships. Specifically, we examine whether there are any differences in how the availability of loans evolved over time for firms that prior to the merger transacted with both of the merged banks, with one of the merged banks, or with none of them. We find the following: (1) Firms that had transacted with both BTM and UFJ saw their borrowing costs increase by 40bp relative to those that had transacted with neither of them. (2) Firms that transacted with one of the two banks saw their borrowing costs increase by a smaller but still significant margin of 20bp relative to those that had transacted with neither of them. And (3) we do not find a significant difference in the extent that borrowing costs increased between firms that transacted with the acquiring bank (BTM) and those that transacted with the acquired bank (UFJ). These results suggest that the bank merger increased firm's borrowing costs partly through the exogenous decrease in the number of firm-bank relationships and partly through changes in the organizational structure of the merged bank, including a consolidation of the branch network.