Federal Thrift Savings Plan's S Fund was the best performing of the $366.7 billion plan's investment options in 2013, with a 38.35% return.
The S Fund, which represents 8% of assets, invests in U.S. midcap stocks and tracks the Dow Jones Wilshire 4500 index.
The next-highest performance came from the C Fund, which tracks the S&P 500 index and returned 32.45% in 2013. It has 24% of assets.
The I Fund, which invests in international equities, returned 22.13% and the F Fund, an aggregate bond fund with 8% of assets, ended the year down 1.68%.
All of the above funds are managed by BlackRock.
The G Fund, an internally managed government securities fund that accounts for 43% of all assets, gained 1.89% in 2013.
Washington-based TSP also has five lifecycle funds that were started in 2005. Their returns for calendar year 2013 ranged from 26.2% to 6.97%.