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The 1966 Carmarthen by-election, was held in Carmarthen, Wales, on 14 July 1966. The contest was significant in that it resulted in the election of Gwynfor Evans, the first ever Plaid Cymru Member of Parliament. Plaid Cymru's victory in the Carmarthen constituency, a seminal moment for Welsh nationalism, was part of a wider process toward Welsh devolution which eventually led to the establishment of the Welsh Assembly in 1999.

Gwynfor Evans' surprise win is credited with laying the foundations for Winnie Ewing's victory for the Scottish National Party at the 1967 Hamilton by-election, an event of equal significance for Scottish nationalism.

Results

1966 Carmarthen by-election^[1]^[2]
Party		Candidate	Votes	%	±%
	Plaid Cymru	Gwynfor Evans	16,179	39.0	+22.8
	Labour	Gwilym Prys-Davies	13,743	33.1	−13.1
	Liberal	Hywel Davies	8,650	20.8	−5.3
	Conservative	Simon Day	2,934	7.1	−4.5
Majority			2,436	5.9	N/A
Turnout			41,506	74.9	−7.7
Registered electors			55,407
	Plaid Cymru gain from Labour		Swing	+12.0

^ "1965 By Election Results". Archived from the original on 29 October 2013. Retrieved 19 August 2015.
^ Craig, F. W. S. (1971). British parliamentary election results 1950-1970 (1 ed.). Chichester: Political Reference Publications. ISBN 9780900178023. Page 570

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