Local newspaper Midi Libre reported that the 20-year-old man was shot dead after trying to steal cannabis that was growing nearby.
"We believe that four young people from Nîmes decided to steal some cannabis plants growing in a field next to the motorway," said the Nîmes public prosecutor, Robert Gelli. "While three of them were in the field, shots were fired."
Gelli said the shots probably came from "the owners of the field." A fourth man had been standing lookout next to the two cars the group had arrived in.
Passing motorists thought the man was a hunter who had been hit. He collapsed on the hard shoulder of the motorway and emergency services were called.
Police quickly rounded up a number of people for questioning on Sunday afternoon but no charges have yet been laid.