Investing in mutual funds for retirement is never too late. And the Zacks Mutual Fund Rank can be an excellent tool for investors looking to invest in the best funds.

How can you tell a good mutual fund from a bad one? It's pretty basic: if the fund is diversified, has low fees, and shows strong performance, it's a keeper. Of course, there's a wide range, but using the Zacks Mutual Fund Rank, we've found three mutual funds that would be great additions to any long-term retirement investors' portfolios.

Let's take a look at some of our top-ranked mutual funds with the lowest fees.

If you are looking to diversify your portfolio, consider Deutsche Science and Technology S (KTCSX Quick QuoteKTCSX - Free Report) . With a much more diversified approach, KTCSX--part of the Sector - Tech mutual fund category--gives investors a way to own a stake in the notoriously risky tech sector. This fund is a winner, boasting an expense ratio of 0.71%, management fee of 0.45%, and a five-year annualized return track record of 12.77%.

Federated MDT Mid Cap Growth R (FGSKX Quick QuoteFGSKX - Free Report) : 0.83% expense ratio and 0.75% management fee. FGSKX is a Mid Cap Growth mutual fund. These mutual funds choose companies with a stock market valuation between $2 billion and $10 billion. With yearly returns of 10.41% over the last five years, FGSKX is an effectively diversified fund with a long reputation of solidly positive performance.

Fidelity OTC Portfolio K (FOCKX Quick QuoteFOCKX - Free Report) . Expense ratio: 0.69%. Management fee: 0.68%. Five year annual return: 13.67%. FOCKX is a Large Cap Growth mutual fund, and these funds invest in many large U.S. firms that are projected to grow at a faster rate than their large-cap peers.

There you have it. If your financial advisor had you put your money into any of our top-ranked funds, then they've got you covered. If not, you may need to talk.