The A26 is closed both ways at Lewes due to a serious accident between a car and a motorbike.
The collission happened between the B2192 Earwig Corner, Lewes, and the A22 Lewes Road (Uckfield).
Sussex Police said the motorcyclist had serious injuries and was taken to the Royal Sussex County Hospital in Brighton.
His condition is stable.
The road remains closed at 3pm.