Abstract: Using a unique proprietary dataset of institutional trades, we show that it is a common practice for mutual fund families to buy and sell the same stock for different accounts on the same day. While many of these trades are executed through the external market, there is also a considerable amount executed by crossing internally within the mutual fund family. Internally crossed trades incur lower implicit costs and explicit costs of trading, and we estimate that the total trading cost savings enjoyed by our sample mutual fund families amount to $1.28 billion during our 12-year period between 1999 and 2010. If mutual fund families are able to exploit profitable opportunities by executing those potentially crossable market trades through an internal crossing mechanism, there can be a further saving of trading costs of $5.65 billion. Since mutual fund families with larger trading value and number of trades are more likely to trade by internal crossing, our findings provide a new explanation for the sources of economies-of-scale in asset management.