A 57-year-old motorcyclist was killed after crashing into a signpost at Oxenhope last night.
Police are appealing for witnesses to the accident to come forward and are "urgently" seeking the driver of a car that was believed to be close by at the time.
The crash happened in Hebden Bridge Road at around 6.20pm.
Police closed the road immediately after the crash, which involved a Suzuki motorcycle, to carry out an investigation.
Officers say inquiries are at an early stage but they are urgently trying to trace the driver of the car, which did not stop at the scene, to help establish the circumstances of what happened.
The Major Collision Enquiry Team (MCET) is investigating.
Detective Superintendent Simon Beldon, of West Yorkshire Police, said: "This was a serious incident and we are appealing for anyone who saw what happened to come forward.
"Initial inquiries are underway to determine precisely what took place. We urgently need to speak to the car driver, believed to be a man, and I would encourage him to come forward now to speak with us.
"This man or anyone who can assist with inquiries should contact the Major Collision Enquiry Team on 101. Anonymous reports can also be made to Crimestoppers on 0800 555 111."