Mid-America Pipeline Company has pleaded guilty to negligently releasing about 200,000 gallons of ammonia into a Kansas creek.
Authorities say the incident resulted in the killing of more than 25,000 fish.
The Delaware company has agreed to pay a $1 million criminal penalty.
A pipeline that the company owned ruptured about six miles west of Kingman in October 2004. More than a million pounds of liquid ammonia was released.
U.S. Attorney Eric Melgren says the ruptured pipe created a vapor cloud that was a half-mile wide and 1.5 miles long. The ammonia flowed into a 10-mile stretch of a tributary of Smoots Creek, killing the fish, including several endangered ones.