Europe's spreading debt crisis has pushed the euro down to its lowest point against the dollar in more than two years.
Spain's borrowing rates spiked again, pushing the country closer to needing a financial lifeline.
Investors worry that if Spain has to be rescued, the rest of Europe will not be able to afford it.
The yield on Spain's 10-year government bond spiked above 7.5%, a sign that investors are worried about the country's ability to pay its debts.
Experts believe that those rates are unsustainable in the long run. Greece, Ireland and Portugal had to ask for emergency financial support after their borrowing rates rose above 7%.
Traders also sold the euro after Moody's ratings agency lowered its outlook for Germany, the Netherlands and Luxembourg, saying those stronger countries might have to give financial aid to Spain or Italy.
The euro fell to 1.2061 dollars from 1.2125 dollars on Monday. The euro fell as low as 1.2041 dollars earlier, its lowest point against the dollar since June 2010.
The pound fell to 1.5504 dollars from 1.5521 dollars.