The hair, which was found attached to duct tape on July 17, was subjected to genetic testing at the FBI crime lab in Quantico, Virginia, and the results showed they did not come from the 18-year-old who has been missing for two months, said spokeswoman Judy Orihuela.
"The results were negative," said FBI spokeswoman Judy Orihuela. "It wasn't Natalee's hair."
A park ranger found the duct tape while collecting trash on Aruba's northeast coast — the opposite side of the island from where the Alabama woman was last seen in public.
Testing was also conducted on a sample of the hair at a crime lab in the Netherlands, but the results were not immediately available.
CBS News Correspondent Kelly Cobiella, who's in Aruba, said the test results give Holloway's family some reason for optimism.
They say it's a relief, and, with Natalee missing two months, they want investigators to go back to week one.
"I just want to go back to the beginning, because there were a lot of clues right at the beginning of this," said Holloway's stepfather, George "Jug" Twitty.
The family is convinced all the answers to Natalee's disappearance are within five miles of the hotel where she was staying, and finding duct tape on the other side of the island simply didn't fit.
"We never really felt like that was something that we were sure about," Jug Twitty says. "We've been through the ups and down of so many different things like that."
Meanwhile, authorities continued to drain the pond across from the Marriott Hotel, near where the U.S. teen was last seen nearly two months ago.
The authorities declined to specify what they were looking for in the pond, but confirmed it was part of the investigation into Holloway's disappearance on May 30, the final night of a high school graduation trip to the Dutch Caribbean island. The job was expected to be finished Wednesday but it's taking longer than expected to drain.