Two teams of air ambulance medics are treating a casualty at the side of the road near Lydiard Park.
Emergency services were called to the scene in Hay Lane at around 8pm.
Police have closed the road in both directions as Wiltshire Air Ambulance and Great Western Air Ambulance attend the serious medical emergency.
The male casualty is being treated by four critical care medics and a number of local ambulance medics.
