Polling started at 7 am in Kishanganj, Katihar, Purnia, Bhagalpur and Banka and will continue till 6 pm, a senior official said.
The fate of 50 candidates is being decided for five Lok Sabha seats in Bihar as polling began amid tight security on Friday morning.
Heatwave conditions were a concern, with temperatures rising above 40 degrees C, so arrangements for tents and drinking water had been made at the polling centres, a senior official said.
Polling started at 7 am in Kishanganj, Katihar, Purnia, Bhagalpur and Banka and will continue till 6 pm, he added.
Over 93 lakh voters, including 45.15 lakh women and 306 belonging to the third gender, will decide the fate of 50 nominees, including three women, in these five seats.
Young voters form more than one-third of the electorate, with 13.77 lakh in the age group of 18-19 years and another 20.86 lakh aged between 20 and 29 years.
Chief Minister Nitish Kumar's JD(U), a BJP ally, is contesting all the five seats, including Kishanganj, which it seeks to wrest from the Congress, while its MPs in the remaining four constituencies are trying to retain their turf.
More than 150 companies of paramilitary forces are assisting the local police in ensuring free and fair polls, the official said.