It was involving a motorcycle and a car near Herstmonceux on Monday, April 18.
It happened at about 8.20pm on the A271 Hailsham Road, between the junctions with Old Road and Under Road.
A 48-year-old rider suffered serious injuries and was taken to the Royal Sussex County Hospital where he remains.
Anyone who witnessed the collision or anyone with relevant footage of the vehicles in the area is asked to contact Sussex Police online or call 101 and quote Operation Jackdaw.
