Wall Street traders are demanding the biggest premiums to buy and sell credit in almost two years as they seek protections from market swings driven by Europe's debt crisis and a slowing global economy.

A measure of the cost of trading credit-default swaps has tripled this month as prices gyrate the most in 13 months, according to data compiled by Bloomberg and CMA in London. Amid the volatility, the biggest bond dealers cut their holdings of corporate securities to $73.1 billion as of Aug. 17, the least since July 2009, Federal Reserve data show.

The surge underscores the fragility of credit markets three years after the collapse of Lehman Brothers Holdings Inc. triggered the biggest corporate bond losses in at least 35 years. With junk-rated securities poised to lose the most this month since November 2008, banks and investors are bracing for broader declines on concern Europe's fiscal imbalances will infect the banking system at a time when the economy may not be strong enough to withstand such headwinds.