The Dow Jones industrial average was down 109 points, or 0.7 per cent, at 16,876 moments as of 11:45 a.m. on Thursday. The Standard & Poor's 500 index was lower by 11 points, or 0.6 per cent, at 1,961. The Nasdaq was off 29 points, or 0.7 per cent, at 4,389.
U.S. markets were hit by earlier stock losses in Europe, where worries over the health of one of Portugal's largest financial groups slammed stocks and pushed investors into safer bonds.
The yield on the 10-year Treasury note slipped to 2.52 per cent from 2.55 per cent late Wednesday. Yields fall when demand for bonds rises.