A Florida judge has dismissed an attempt to prosecute Greenpeace for protesting against mahogany shipments.
The charges arose from a protest over illegal logging
The environmental group was charged in 2002, after its activists boarded a ship off Miami that carried illegally-felled mahogany from the Amazon.
The US attorney-general's office argued that their actions broke a 19th century law designed to stop prostitutes tempting sailors off ships.
The judge threw out the prosecutors' case for lack of evidence.
Greenpeace had argued that the prosecution was designed to muzzle free speech.
After Wednesday's decision by a federal judge in Miami, a Greenpeace lawyer said: "It's a message that the government can't just throw any charge at an organisation to silence them."
If found guilty, the group would have had to undergo monitoring.
Prosecutors had based their case on a 1872 law originally aimed at preventing boarding-houses from luring sailors away from their ships with offers of prostitutes, strong drink and warm beds.
The law was last enforced in 1890.