Kilgoris MP Gideon Konchella defeated his rival in the Jubilee party primaries by a narrow margin.
Mr Konchella garnered 19,531 votes against Joel Tasur, who got 19,065 votes.
Another aspirant Samuel Cheruiyot was third with 1,342 votes.
Mr Tasur rejected the results claiming the exercise was marred by irregularities.
He vowed to challenge the results at the party's tribunal.
“I am dissatisfied with the outcome of the voting. I am aware that arbitrary cooking of figures was done in some polling stations, particularly in areas deemed as strongholds of the incumbent,” he said.
Mr Konchella will face Mr Richard Kelele of CCM and Peter Morintat of ODM in the August elections.
Mr Morintat, whose victory had initially challenged is now breathing easy after the ODM party disputes tribunal upheld his victory.
“I am happy that the tribunal upheld my victory and we are now seeking the support of all party stakeholders to enable me to beat Mr Konchella,’ he told the Nation on Wednesday.
In Emurua Dikirr constituency, Mr David Keter won the Jubilee ticket after garnering 12, 180 votes, beating his closest rival Mr Paul Tunoi who got 5, 883 votes.
Mr Keter will take on incumbent Johanna Ng’eno of CCM in August elections.
Emurua Dikirr constituency returning officer Naikanai Kuluo said the votes from Mogor polling station in Mogondo ward were not tallied due to lack of consensus on the correct figures between the agents of various aspirants.
“The county returning officer will decide on what to do about this hitch,” he said.