Waikato police are seeking witnesses of a "serious" crash that occurred on State Highway 39 south west of Hamilton on Thursday afternoon.
A motorcycle and car collided near Rice and Allcock Roads just after 2pm.
The rider is in critical condition in Waikato Hospital and the driver of the vehicle is uninjured.
A Waikato man, who was fourth on the scene, said the motorcyclist was travelling south and was riding a registered dirt bike.
Police are seeking witnesses, particularly the driver of a small silver hatchback.