The G Fund, invested in government securities, was the only one of the five to grow in both months. It had a 0.29 percent return in April and a 0.39 percent return in May.
The F Fund, on the other hand, fell both months -- by 2.54 percent in April and another 0.5 percent in May. The fund is made up of fixed-income bonds.
The C Fund, which consists of common stocks, rebounded in May with a 1.37 percent gain.
The S Fund, which invests in the stock of small and mid-size companies, posted the most significant losses in April -- 3.94 percent -- and the biggest gains in May -- 1.5 percent.
The I Fund suffered a 2.31 percent loss in April and earned the smallest gain in May growing by 0.3 percent. The fund invests in international stocks.
During the past 12 months, all but the F Fund have had positive returns. The I Fund had the largest net earnings with a nearly 32 percent gain over the past year. The G Fund had the smallest gain at 4.1 percent.