“He gained weight, developed a fuller and shinier coat, and demonstrated strong overall health during follow-up examinations.”
The fox weighed about 11lb (5.1kg) when he arrived at the zoo and now weighs nearly 14lb (6.2kg), staff said.
Craig Piper, interim director, said: “Basil has done exceptionally well since arriving at the Bronx Zoo.
“We are very pleased with his recovery and continued progress since his long journey across the Atlantic.”
The zoo said it was still assessing a suitable long-term home for the fox.
A spokesperson for Associated British Ports Southampton previously joked the animal had “booked itself a transatlantic crossing”.
The case came weeks after a stowaway raccoon discovered on a cargo vessel in Southampton was put down because of concerns it could be carrying diseases including rabies.