(Bloomberg) — Fidelity Investments, the largest provider of 401(k) retirement savings plans, says average balances in the employer-sponsored accounts reached a record high in 2012 as market gains boosted assets.

The average U.S. balance rose 12% from 2011 to $77,300 as of Dec. 31, 2012, according to a report this month by the Boston-based mutual-fund firm. Fidelity administers plans for about 12 million workers and has been tracking the data since 2000.

However, it’s important for retirement plan sponsors to note that two-thirds of the average-balance increase came from market appreciation, as the S&P 500 gained 13% last year — only one-third can be attributed to participant contributions, meaning there’s more work to be done in the area of retirement education and communication.

The year-end average was 58% higher than in 2008, when the global financial crisis shook equity markets. That year the average balance plummeted to $48,900 as the S&P 500 Index declined 38 percent. Balances have since rebounded in three of the past four years. The previous record high was $75,900 at the end of 2012’s third quarter.