After casting his vote, a beaming Adityanath told reporters that the "BJP continues to get a groundswell of public support since they are witnessing development, peace and prosperity under its government".
The SP candidate from Phulpur was ahead of BJP candidate Kaushalendra Patel.
Meanwhile, results continue to pour in from other seats. The seats were vacated by Chief Minister Yogi Adityanath and his deputy Keshav Prasad Maurya of the BJP and today's by-election is a prestige battle for the ruling party, which took on the combined might of regional heavyweights Samajwadi Party and Mayawati's BSP.
Bihar bypolls are being seen as a litmus test for Chief Minister Nitish Kumar after he pulled out of the state's Grand Alliance and joined the BJP.
Demographics in this constituency favors the opposition combine due to the presence of almost 2.5 lakh voters belonging to the Muslim community.
However, a large chunk of the Muslim votes, which used to traditionally go to the SP, is expected to go to independent candidate and local strongman Atiq Ahmed this time, helping the BJP. The Congress too has fielded a candidate. Bihar by-polls 2018 for the Araria seat was necessitated by the death of RJD MP Mohd Taslimuddin. In Gorakhpur seat in Uttar Pradesh, there was a voter turnout of 43% and in Phulpur, 37.4% of the electorate turned up to vote. As of the latest numbers, the BJP is leading by about 6,500 votes with 1,03,000 votes.
The elections are keenly watched as the RJD is fighting alone, while its former partner JDU has shifted its alliance to the BJP's National Democratic Alliance.
Initially, the postal ballots were being counted after which the electronic voting machines (EVMs) will be opened for counting.