When you think 401(k), does Fidelity come to mind? It should. Fidelity is the largest provider of 401(k) retirement savings plans and the biggest mutual fund firm in the United States. But with an eye-popping $1.3 trillion in assets under management, the company remains closely held, with the founding Johnson family controlling the Boston-based firm. With the stock market performing well lately and more people looking toward retirement, it's easy to see why an IPO would be quite attractive. But pricing the company would be tough, warns Taulli, noting the dearth of other pure-play mutual fund firms out there.