A woman was attacked by a man after confronting a girl riding a motorbike on a nature reserve.
The woman in her 50s challenged the girl last Saturday at about midday.
A man with the girl assaulted her before riding off on the bike with the girl.
The woman was left with minor injuries.
Police are now appealing for witnesses.
The suspect is described as a white man, around 5ft 8in and in his 30s, with cropped hair under a baseball cap, no facial hair and a light-coloured hoodie with grey jogging bottoms.
Anyone who witnessed the incident or has any information that could help is asked to contact police online or via 101, quoting serial 504 of 25/11.
Off-road bikers have been plaguing the valley for many months now.
Last month, Sussex Police told Brighton and Hove News it was arranging a meeting with ward councillors, residents to discuss the issue.
A spokesman said: “Police are aware of reports of anti-social motorcycle riding around the Sheepcote Valley area and are working closely with the local authority to find an appropriate, long-term resolution.
“We will continue to maintain a visible presence in the area and engage with the local community.
“A community meeting, alongside local ward councillors, is due to be arranged in the coming weeks for residents to discuss any issues.”
