President Bush has congratulated the Congolese people for what he called "their deep desire to embrace democracy" - because of the country's constitutional referendum.
A White House spokesman, in a statement, said the referendum represents an important step in the democratic process in the Democratic Republic of Congo.
Polls closed Monday after a second day of voting on a new constitution designed to end years of war and chaos.
Officials say turnout was high for the referendum. Final results are not expected for several days.
If approved, the constitution would pave the way for Congolese elections next year.
The DRC is struggling to emerge from decades of political instability and violence. The country's most recent conflict, which ended in 2003, drew in armies from six neighboring states and left nearly four million people dead.