The 1878 Carmarthen Boroughs by-election was a parliamentary by-election held for the House of Commons constituency of Carmarthen Boroughs in West Wales on 11 May 1878.
The by-election was caused by the resignation of the sitting Liberal MP, Sir Arthur Cowell-Stepney, 2nd Baronet.
The Liberals selected Benjamin Thomas Williams, QC, a successful barrister and a former Recorder of Carmarthen.
There being no other candidates putting themselves forward Williams was returned unopposed.[1] ----