Treasury price rose Thursday after Saudi Arabian police opened fire on protesters.
Signs of global turmoil and economic weakness often send traders into the safety of U.S. government debt. Traders had already pushed Treasury prices slightly higher in the morning on news that more people filed unemployment claims for the first time last week.
Protesters have already toppled governments in Tunisia and Egypt. Traders have been concerned that an uprising against Saudi Arabia, the world's largest exporter of crude, could threaten its oil production.
The 10-year note is up 90.6 cents per $100 invested. The higher price lowered the yield to 3.37 percent from 3.47 percent late Wednesday.
The Treasury also sold $13 billion in 30-year bonds at a yield of 4.56 percent. Bidding was stronger than the recent average.
The price of the 30-year bond jumped $1.81, sending the yield down to 4.50 percent from 4.61 percent.
The yield on the two-year note fell to 0.63 percent from 0.70 percent.
The yield on the three-month T-bill fell to 0.06 percent from 0.09 percent. The discount was 0.07 percent.