This paper estimates the short-run impact of the Pension Protection Act of 2006 (PPA2006) on holdings of employer stock in defined contribution pension plans. PPA2006 allowed participants in plans with employer stock to diversify their holdings. However, stand-alone ESOPs, i.e., those that do not allow employee elective deferrals or after-tax contributions, were exempt from this provision. Using detailed Form 5500 financial data for stand-alone ESOPs and those that allow employee elective deferrals or after-tax contributions, so-called KSOPs, from 2003-5 (before) and 2007-9 (after) the PPA and a quasi-experimental empirical framework, two primary empirical findings emerge. First, the share of plan assets in company stock fell 7 percentage points for KSOPs, because of the diversification provisions in PPA2006, a substantial decline. There was no change in holdings for stand-alone ESOPs. Second, most of the decline occurred in plans that had between 25-50 percent of plan assets in employer stock. Nonetheless, in 2009 still two-thirds of KSOPs had more than 10 percent of assets in company stock, the statutory limit for defined benefit pension plans.