Shares of troubled US airline Northwest lost more than half their value in Thursday trading on Wall Street.
The US airline industry is a tough marketplace at present
Northwest shares, quoted on the Nasdaq index, fell 99 cents, or 52%, to $0.99 as investors reacted to its filing for bankruptcy protection on Wednesday.
Rival airline Delta, which has also filed for Chapter 11, fared better as its shares fell just 5.6% to 75 cents.
Delta and Northwest aim to join United and US Airways, which are already operating under Chapter 11 protection.
Chapter 11 gives a US company time to rearrange its finances while still conducting business.
The US airline industry - recently hit by high fuel costs and tough competition - has failed to recover from the post-11 September 2001 downturn.
Delta is the third-largest airline in the US by revenues and Northwest is the fourth.
Bankruptcy speculation has dogged both Delta and Northwest for months.
Delta has lost $10bn (£5bn) since 2001 and is saddled with $14bn in debt.
LARGEST US CARRIERS
6. US Airways
By revenue. Source: Fortune 500
Minnesota-based Northwest's problems are focused on high labour costs, which it is trying to cut by $1.1bn.
Delta and Northwest are known as "legacy" airlines in the US, as they are two of its long-established, traditional flyers.
In Europe such airlines are generally known as "flag carriers", as they were historically state-owned.
On both sides of the Atlantic, such airlines have faced increased competition from a new generation of low-cost carriers in recent years.
This additional competition, in combination with higher fuel costs and reduced passenger numbers after the 11 September attacks, has caused financial difficulties for those legacy airlines that have not been able to reform quickly enough.