A 60-year-old cyclist was found lying in the road with head injuries.
He was discovered at about 8am on Saturday in Upper Duke's Drive, Eastbourne.
The man was taken to Eastbourne District General Hospital where his injuries were described as serious.
Police are trying to find out how he was injured and are asking any witnesses who saw a collision to contact PC Gavin Spencer at Sussex Police's road policing unit at Polegate on 0845 6070999.
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