A motorbike rider was airlifted to hospital at around 5.30pm today after he came off his machine about a mile from the Beckhampton roundabout.
Police have tweeted that one lane of the road will remain closed this evening while investigations continue.
A Beckhampton resident was heading to Devizes when he passed the accident on the A361.
He said no other vehicle was involved and the rider appeared to be heading from Devizes to Beckhampton.
The victim was on the verge, his bike on its side in the road, while another motorcylist and police in an unmarked BMW were tending to the rider.
The witness said that 45 minutes later, on their return journey from shopping in Devizes, the biker remained on the verge, receiving aid from paramedics from the air ambulance that had landed in a field nearby.
Some 30 minutes later he heard the helicopter lift off.
Sgt Clare Wallace tweeted later: "Just had an update from hospital that the rider is doing well. We wish him a speedy recovery."