I study broker-dealers’ trading activity in the US corporate bond market. I find evidence of market making and of proprietary trading exploiting possible mispricings. Market making occurs when customers both buy and sell a bond in a day, which happens half of the time: as predicted by market making theories with adverse selection or inventory costs, prices go down (up) as customers sell (buy). Otherwise, evidence is in favor of broker-dealer initiated trades, i.e. proprietary trading: prices go up (down) when customers sell (buy). I test one aspect of proprietary trading predicted by theories of limits of arbitrage: dealers buy (sell) bonds that are relatively cheap (expensive) with respect to bonds of similar maturity, or with respect to T...