Greensborough Highway Patrol police are appealing for witnesses following a serious collision in Thomastown this morning.
It is understood a Mitsubishi utility was performing a u-turn at Dalton Road when the vehicle collided with a motorcycle about 7.30am.
The motorcyclist, a 22-year-old Diamond Creek man, was taken to hospital with life-threatening injuries.
The driver of the car, a 23-year-old Roxborough Park man, stopped at the scene.
He has subsequently been arrested and is assisting police with their enquiries.
Anyone who witnessed the collision, has dash cam footage or any information that could assist investigators is urged to contact Crime Stoppers on 1800 333 000 or submit a confidential report via www.crimestoppersvic.com.au