Incomplete results released by the election organisers on Sunday indicated a strong backing for the three ruling parties in about 140 races for municipal councils and mayors in Muslim-Croat and Serb regions.
The long-dominant Serb Democratic Party (SDS), founded by fugitive war crimes suspect Radovan Karadzic, appeared to have lost some ground to the opposition Alliance of Independent Social Democrats (SNSD) in the Serb Republic.
A victory for the main parties in Saturday's election was largely expected and would not surprise the international community assisting Bosnia's recovery after the 1992-5 war, which for the first time did not favour fractured moderates.
After ceding some ground in local polls and losing a general election in 2000, the nationalists won the 2002 general vote.
They have since pledged to carry out reforms and establish closer links with NATO and the European Union.
But the Serbs' failure to catch indicted war criminals and a dysfunctional state are obstacles to this.
Voter turnout fell below 50% for the first time, with 55% of 2.3 million registered voters ignoring the vote, election commission member Lidija Korac said.
There was a low turnout in the
third post-war local elections
"This may be the result of voter fatigue and a complicated registration process, but the voters may also be punishing those in power or they simply don't believe things can change," Korac said.
The low turnout confirmed a prediction by diplomats, analysts and opinion polls that many Bosnians would prove to be disillusioned with unfulfilled promises. Urban and young voters were the largest groups abstaining from voting, boosting the nationalists' chances.
The Croatian Democratic Union (HDZ) is expected to remain dominant in Croat areas.
The Party of Democratic Action (SDA) is expected to return to power in some of 15 mainly Muslim cities, where the opposition Social Democratic Party (SDP) won in 2000.
The SDS is expected to stay strong in traditionally hard line eastern parts of the Serb Republic. But the SNSD is set to add to 10 municipalities it controls in western parts and is poised for a huge win in the biggest city of Banja Luka.
Full preliminary results will be released late on Sunday.