The Ulster Unionist leader, David Trimble, has lost his seat in Upper Bann to the DUP's David Simpson.
Conceding defeat, Mr Trimble thanked his family and party members for their support.
He also thanked the people of Upper Bann who supported him over the past 15 years. Mr Trimble said that he was proud of his record and that it would endure.
Since he was elected Ulster Unionist leader in September 1995, Mr Trimble has survived countless efforts to topple him.
Mr Trimble's position as leader of the Ulster Unionists is now certain to come under scrutiny.
The Ulster Unionist Chairman, James Cooper, said the party was bitterly disappointed at the result, which raised a number of questions at a strategic and political level within the party.
Meanwhile, the leaders of the two other main parties have been elected with big majorities - Gerry Adams in Belfast West and Ian Paisley in North Antrim.
While the SDLP leader, Mark Durkan, has retained the seat formerly held by John Hume in Foyle.
105 candidates were contesting the North's 18 Westminster seats. In the local government elections, 918 people are contesting 582 council seats. Ballots for the local government elections will be counted over Monday and Tuesday.
Results are on rte.ie/news/electionsni2005 .
