Exchange traded funds have yet to make a dent in the trillion dollar 401(k) plan industry, dominated by mutual fund heavy weights. While new disclosure rules may help ease ETFs into more plans, there are some obstacles to overcome.

When the average employee understands more about performance and the fees they’re paying, you’ll start seeing more plan providers, especially smaller ones, include ETFs. But there are some plumbing issues in getting ETFs into 401(k) plans.

More notably, smaller 401(k) plans hold class A-shares mutual funds that have higher fees than ETF equivalents, reports Rachel Louise Ensign for The Wall Street Journal.

However, larger retirement plans include institutional mutual fund shares, which may cost less than some ETF offerings. As such, big plan sponsors, like Fidelity and Vanguard, have shied from offering ETFs in their 401(k)s.

“When ETFs were newer in the marketplace, we had plan sponsors looking interested in the product. They’ve begun to get more informed and understand the products better and understand the way that low-cost mutual funds serve the same purpose,” Beth McHugh, a Fidelity vice president, said in the article.

Additionally, mutual funds are traded as one at the end of the day through a clearinghouse, which charges next to nothing. ETFs, though, are traded through a brokerage firm, which incur commissions on per-share basis. As it stands, the intra-day tradability of ETFs leaves plan participants with higher commission fees.

Greg Carpenter, chief executive officer of Employee Fiduciary, also points out that ETFs are traded in full shares. Employees may have money that goes uninvested if they do not meet the dollar per share prices. Although, plan providers are working to offer partial shares on ETFs for clients.

Nevertheless, industry observers point to changes in Fidelity that could result in more ETF offerings and force other large competitors to follow suit. Notably, Fidelity’s recent hiring of ETF exeuctive Anthony Rochte from State Street Global Advisors. [Fidelity Hires State Street ETF Exec Rochte]

“”It is very clear they are going to step up their game in ETFs,” James Polisson, managing director of ETFs at Russell Investments, said in the article. “That likely includes 401(k)s.” Fidelity declined to comment on the specifics of Mr. Rochte’s new role.”

ING Direct ShareBuilder 401(k) plan only offers ETF 401(k) plans. Charles Schwab is planning to expand its offerings. TD Ameritrade also provides registered investment advisors ETF and mutual fund options for their employees. [ShareBuilder Expands Its All-ETF 401(k) Plan]

For more information on ETFs in 401(k)s, visit our 401(k) category.

Max Chen contributed to this article.