A cyclist, 18, is fighting for life after colliding with a truck in Byford.
The crash happened on Thomas Road, near the intersection of Tonkin Highway, at about 11.20am.
The teenage rider, from Forrestdale, was dragged under the truck and suffered serious chest and head injuries.
He was treated at the scene by St John paramedics before being flown by the RAC Rescue helicopter to Royal Perth Hospital in a critical condition.
Police said the 46-year-old truck driver from Piara Waters in Armadale was assisting with inquiries.
No charges have been laid.
Police want to speak to anyone who saw the crash, or who may have seen the truck or the teenager on a motorised bicycle before the crash.
Anyone with information is asked to call Crime Stoppers on 1800 333 000.