Abstract We argue that bank holding companies (BHCs) extend shadow insurance to the prime institutional money market funds (PI-MMFs) they sponsor and that PI-MMFs price this shadow insurance by charging investors significantly higher expense ratios and paying lower net yields. We provide evidence that after September 2008, expense ratios at BHC-sponsored PI-MMFs increased more than at non-BHC-sponsored PI-MMFs. Despite higher expense ratios, BHC-sponsored PI-MMFs did not experience larger redemptions than non-BHC-sponsored PI-MMFs. In addition, we show that expense ratios increased with BHCs’ financial strength and the likelihood of their support; however, this expense ratio differential disappeared after the 2016 MMF reform. (JEL G2, G21, G23, G28, H12, H81)