Lincolnshire Police are seeking to identify the man in this CCTV image after an assault in Lincoln city centre on September 26, 2011.
The assault took place just after midnight on Silver Street.
A 19-year-old local man got into an argument with the offender, who punched him, knocking him unconscious and causing bruising.
Anyone who recognises the man in the image is asked to call PC Michael Fearn at Lincoln Police Station via the 101 non-emergency number.
Alternatively, you can call independent charity Crimestoppers anonymously on 0800 555 111.