A teenage motorcyclist from Seaford was airlifted to hospital after he was seriously injured in a collision on the A267 south of Horam.
Sussex Police said the collision took place at about 9.30am on Monday 6 May, when the motorcyclist, aged 18, was travelling north.
He was taken to Kings College Hospital in London for emergency treatment.
Police are keen to speak to a potential witness driving a red vehicle. It was travelling south at the same time.
All witnesses are asked to contact Sussex Police on 101 quoting Operation Channel.