A 15-year-old boy was hospitalised after colliding with a motorbike.
The accident happened on the A270 Old Shoreham Road, Southwick, near the road's junction with Upper Kingston Lane.
It occurred on Wednesday just before 8pm.
The Sussex air ambulance turned up but both casualties were eventually taken to hospital by road ambulance.
The boy, from Hove, suffered serious ankle injuries and was taken to the Royal Sussex County Hospital in Brighton for treatment.
The motorcyclist, a 20-year-old Shoreham man riding a Yamaha 125, was taken to Worthing Hospital with bruising but was not detained.
Anyone who saw what happened is asked to email firstname.lastname@example.org or phone Sussex Police on 101, quoting Operation Lanark.