States Bristle as Investors Make Wagers on Defaults

As U.S. cities and towns wrestle with financial problems, investors are finding a new way to profit on their misery: by buying derivatives that essentially bet municipalities will default.

These so-called credit default swaps are basically insurance contracts that have long been available to protect holders of corporate bonds against default. They became available a few years ago for municipal debt, allowing investors to short sell—or bet against—countless cities, towns and bridges, and more than a dozen states, including California, Michigan and New York.

The derivatives are still thinly traded, but their existence has the potential to make investors skittish ...