The polls are open to voters until 2200 GMT on Thursday
Voters are to go to the polls in Glenrothes to choose a new MP.
Labour is defending a majority of 10,664 in the Fife constituency which was made vacant by the death of John MacDougall in August.
The polls open at 0700 GMT and close at 2200 GMT. The result should be known by the early hours of Friday.
The turnout at the last election in 2005 was 56.1%. The seat borders Prime Minister Gordon Brown's Kirkcaldy and Cowdenbeath constituency.
Although the parliamentary constituency is called Glenrothes it includes a number of other towns and villages stretching from Auchterderran and Cardenden in the west, to parts of Leven in the east.
The constituency population was estimated at 88,351 in 2006.
There are 49 polling places in the constituency and 95 boxes have to be counted.
Labour won the seat in 2005 with 19,395 votes, followed by the Scottish National Party on 8,731, the Liberal Democrats on 4,728 and the Conservatives on 2,651.
Morag Balfour, Scottish Socialists
Maurice Golden, Conservative
Peter Grant, SNP
Lindsay Roy, Labour
Kris Seunarine, UKIP
Harry Wills, Liberal Democrats
Louise McLeary, Solidarity
Jim Parker, Scottish Senior Citizens Unity Party
Labour selected Lindsay Roy, the 59-year-old rector of Kirkcaldy High School, as its by-election candidate.
Peter Grant, leader of Fife Council, was nominated to contest the seat for the Scottish National Party.
The Conservatives chose 28-year-old Maurice Golden, an environmental campaign manager for Keep Scotland Beautiful, while businessman Harry Wills campaigned for the Liberal Democrats.