Richard K Root, 68, an infectious disease specialist, was on a wildlife tour of the Limpopo River in remote northeastern Botswana on Sunday when he was snatched.
Dr Root was in a lead dugout with the tour guides when the crocodile rose out of the water and grabbed him. He was not seen again.
The tour guides were on the look out for hippos, but there had been no reports of crocodile attacks in the area.