Police are looking for a tanker driver who witnessed a crash on the A27 last night.
The collision, involving a Ford Transit and a Kawasaki motorcycle, happened just after 6pm near Segensworth.
The motorcyclist, a 53-year-old man, was seriously injured, but the van driver did not suffer any injuries.
Witnesses told officers a man driving a heavy good vehicle, described as a tanker, stopped but only stayed for a short time before driving away.
Officers believe the man may have witnessed the crash and could have important information.
Anyone with information is asked to contact Hampshire Constabulary on 101 quoting incident number 735.