Employers who want to help their employees improve retirement savings are missing out on a big chance to do so: Roth 401(k)s.

That’s according to consultant Willis Towers Watson, which said that employers aren’t educating their employees on the benefits of Roth 401(k)s—which it says can be one of the most tax-effective ways for employees to save for retirement.

Employees can benefit from such accounts for a number of reasons. Roth 401(k) contributions are made with aftertax dollars, resulting in tax-free withdrawals at retirement. In addition, contributions to a Roth account can provide retirees tax diversification when they start to draw on pensions, employer-match contributions from regular 401(k) plans and other taxable forms of income.

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Employees aren’t obligated to take minimum distributions from Roth 401(k) contributions at age 70½, either, unlike regular 401(k)s and Social Security—as long as they’re rolled over to a Roth IRA.

More plan sponsors offer their employees access to Roth 401(k)s, since plan providers and payroll companies have made them less difficult to administer. A Willis Towers Watson survey found that the percentage of plan sponsors offering a Roth option has increased from 46 percent in 2012 to 54 percent in 2014.

But employees aren’t using them; less than 10 percent of plan participants who have the option currently make Roth 401(k) contributions. Why not? According to Willis Towers Watson, a lack of employer-provided education on their benefits is responsible.

“Unfortunately, many employers have yet to appreciate and, in some cases, communicate effectively, the full potential of the Roth option,” Kevin Wagner, a senior retirement consultant at Willis Towers Watson, said in a statement.

Wagner added, “When employees bypass Roth 401(k) contributions, they unnecessarily narrow their tools for tax-effective retirement savings. For some, using distributions before age 65 may enable qualification for health care subsidies under the Affordable Care Act. Roth contributions can also help some retirees avoid the tax torpedo, which can increase a retiree’s marginal tax rate based on the tax phase-in on certain Social Security benefits.”

The consultant suggested some actions employers can take to correct this. They should communicate the features and benefits of Roth 401(k) programs clearly and regularly; use an interactive approach to reach employees; provide effective retirement-planning modelers; and put in place a process that monitors employee financial well-being on an ongoing basis.