A 57-year-old motorcyclist was killed when he crashed into a road sign in Oxenhope last night.
Police have appealed for witnesses to come forward, and are "urgently" looking for the driver of a car that was on the scene at the time.
The crash happened in Hebden Bridge Road at about 6.20pm. The motorcyclist was riding a Suzuki.
The car did not stop at the scene and officers in Major Collision Enquiry Team want to trace the driver to help establish the circumstances of accident.
Detective Superintendent Simon Beldon, of West Yorkshire Police, said: "This is a serious incident and we are appealing for anyone who saw what happened to come forwards.
"Initial inquiries are under way to determine precisely what took place but at this stage it is believed a car was close by when the incident occurred.
"We urgently need to speak to the driver, believed to be a man, to help establish what took place and I would encourage him to come forwards 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."