Thanks primarily to 2012's rising stock market, Fidelity Investments saw its average 401(k) balance reach a record high, the company reported today.

Its average 401(k) balance hit $77,300 for the fourth quarter, up 12 percent from $69,100 at the end of 2011. The quarter's average balance also topped 2012's third-quarter record of $75,900.

Fidelity noted about two thirds of the 2012 jump came from market increases. In 2012, for example, the S&P 500 index was up about 13 percent and the Dow Jones Industrial Average, more than 7 percent. The other one-third of the increase in the average balance came from participant contributions, Fidelity said.

“It’s encouraging to see how continued savings combined with a healthy equity market have led to another record-high balance for 401(k) savers,” said James MacDonald, Fidelity's president of workplace investing.

Participants in Fidelity's 401(k) plans on average defer 8 percent of their annual salaries in the plans. When the typical employer contribution is factored in, whether it's a match or profit sharing, the average participant’s total savings rate increases to 12 percent. In addition, for a 15th straight quarter, more participants increased their savings rate than decreased it (5.8 percent vs. 3.1 percent).

Fidelity noted that interest is growing in Roth 401(k) accounts, especially since new rules have been enacted that allow the balances in traditional plans to be moved to an employer-sponsored Roth 401(k). Today, 37 percent of Fidelity's workplace retirement plans offer a Roth savings option, up from 12 percent five years ago, and of these plans, 12 percent offer the Roth in-plan conversion option.

Younger investors tend to utilize Roth 401(k) plans the most, Fidelity says. One in 10 participants in their twenties contribute to this option, versus 6 percent overall, Fidelity said. Roth contributors also boast a higher savings rate, deferring an average of 11 percent, the company added.