Tories made their first council by-election gain from Labour since May's change of government.
Their candidate David Craggs won at River ward, Medway Council, where representation had been split at the last main local authority polls in 2007.
The result confirms the General Election trend of a huge swing to the Conservatives in Kent which left Labour without any MPs in the county.
The contest was caused by the resignation of Bill Esterson, who was elected to the Commons on May 6 at Sefton Central, Merseyside.
Medway Council - River: C 617, Lab 544, Lib Dem 104, Green 45, BNP 39, English Democrats 33. (May 2007 - Two seats Lab 791, C 770, Lab 712, C 660, Lib Dem 161, Ukip 131, Lib Dem 127, Medway Independent Party 119). C gain from Lab. Swing 3.6% Lab to C.Reuse content