Additional chief electoral officer R. Lakshmanan said voting was peaceful in the state despite fears of violence.
"So far, polls have been peaceful with no report of violence," he said.
Long queues of women and mostly first-time voters were seen since morning, particularly in rural areas.
Voting is being held in Kishanganj, Purnea, Araria, Katihar, Supaul, Bhagalpur and Banka constituencies.
Over 10 million electors will decide the fate of 108 Lok Sabha candidates and 10 assembly hopefuls.
This is the third round of polling in the state that sends 40 members to the Lok Sabha.
Tarique Anwar of the Nationalist Congress Party, Shahnawaz Hussain of the Bharatiya Janata Party and Mohammad Taslimuddin of the Rashtriya Janata Dal are in the fray.
Bihar's most backward pockets with high poverty and illiteracy are part of the five constituencies - Kishanganj, Purnea, Araria, Katihar and Supaul - known as Seemanchal region as they share the border with Nepal, Bangladesh or West Bengal.
It has a sizable Muslim population - from 17 percent in Supaul to nearly 70 percent in Kishanganj.