The 49-year-old British-Australian had planned to reach the Florida Keys late Sunday but her support crew said she ran into an opposing current that made it impossible to continue.
"Penny Palfrey had to be pulled out of the water, after swimming for more than 40 hours, at approximately midnight tonight due to a strong southeast current that made it impossible for her to continue her swim," the Palfrey team said in a Facebook statement. "Penny is presently on her escort boat being taken care of by her crew."
Palfrey was attempting to become the first person to swim the 103 miles from Cuba to the Keys without assistance. Susie Malroney made it in 1997 inside a floating shark cage, but Diana Nyad was unsuccessful in two attempts last year..
She hit the water Friday morning and plugged away against a hot sun, jellyfish and the threat of sharks. CNN said there was no announcement as to how many miles she had covered before calling it quits, although she had passed the 86-mile mark Saturday night.