Shares of AMR Corp, the parent of American Airlines, dropped 80 percent to 32 cents, with more than 125 million shares traded after the company's bankruptcy. It ranked as second-most active early on Tuesday afternoon behind Bank of America.
The stock was halted repeatedly in the early hours of trading due to volatility. A stock in the Russell 1000 index that moves more than 10 percent in a five-minute period is halted under circuit breaker rules adopted by U.S. exchanges after the May 2010 "flash crash."
With a stock at such a low price as AMR, it only takes a move of a few cents to hit the 10 percent threshold that triggers a halt.
The quirk here is that AMR is still a member of the Russell 1000 where the 10 percent level holds. Shares not in that index traded at $1 a share or less are not halted until they move by 50 percent, according to the Securities and Exchange Commission.
While stock volume in AMR is off the charts, the option volume is relatively muted, on track for 40,000 contracts by the close, or about twice the recent average daily volume.