To expound on audreyh1's excellent response, every financial instrument purchased through your broker (including brokered CDs) float in value from day to day depending on how interest rates are behaving. T-Bills are exceptionally liquid and can be sold in the secondary market at any time. The current value you see on Fidelity is approximately what you could receive if you decided to sell the T-bill at that time. If you plan to hold to maturity, the day-to-day fluctuation in the market price of the T-Bill is unimportant. You will simply get the face (par) value at maturity.