Slightly more than half of the nation's nearly six million small businesses may not have the retirement plan that best fits their needs, according to a Fidelity Investments survey of small business owners.

The Fidelity survey found that many small business owners are struggling to understand the features and benefits of their current retirement plans. Some of them are also struggling with the costs.  Among the estimated 100 small business owners who indicated that they currently don't provide a retirement savings plan, 34 of them listed "too expensive" as their reason.

Small business owners were surveyed to gauge their understanding of SEP-IRAs, SIMPLE-IRAs and Self-Employed 401(k) plans. The results highlight the education gap that many small business owners face with their current plans.

When asked a series of basic questions about their retirement plan features and benefits, respondents on average answered just 66 percent correctly.

Half of those who use a Self-Employed 401(k) did not know the maximum annual employer contribution allowed for this plan.

Nearly two-thirds (60 percent) with SIMPLE-IRAs were not aware that employer tax filing is not required.

More than half (55 percent) using a SEP-IRA did not know that the plan is funded solely by employer contributions.

Fidelity's Small Business Owner Retirement Plan study was based on an online survey conducted among 638 small business owners with fewer than 100 employees. Research firm ORC International conducted the online survey in December.