The group was overrun by the avalanche on Wednesday afternoon at an altitude of about 2,500m in the the Gran Paradiso National Park, an Alpine area north of Turin and near the French border.
Tiziano Trevisan, a spokesman for Aosta's hospital, said the guide was in a critical but stable condition in the intensive care unit after suffering abdominal and chest trauma.
"He is conscious, but hasn't said anything," Trevisan said. "I don't think he's been told about the others."
Massimo De Michela, a rescue official in Turin, said the five climbers had been found at the bottom of a gorge having been dragged about 150m by the avalanche.
Rescuers used the helicopter to retrieve the bodies and brought them to a hospital in Cuorgne.
The members of the group were aged between 38 and 60, and most were from the Paris area.
The avalanche descended from Punta Basei, a 3,300m peak, where fresh snow had recently fallen, De Michela said.
He said rescue efforts began after owners of the lodge where the group was supposed to spend the night raised the alarm.
The six mountaineers were hiking more than 12km between two mountain lodges in the Gran Paradiso massif, straddling Italy's Piedmont and Valle d'Aosta regions.
"It's a beautiful hike," De Michela said. "It's a pity it ended so badly."