Members of the ruling United Russia party, once led by Russian President Vladimir Putin, voted with the nationalist Liberal Democratic Party to expel opposition leader Gennady Gudkov from the lower house of the Russian Parliament.
He told lawmakers the action against him was likely a response to his criticism of the Kremlin.
"They're expelling me from the Duma because they're afraid of the truth, afraid of criticism," he was quoted by Russia's state-run news agency RIA Novosti as saying.
Opposition voices in Russia have grown louder since State Duma elections in December. Mass protests erupted when Vladimir Putin won a third non-consecutive term as president early this year. Duma elections, which saw the ruling United Russia party secure its dominance, and presidential elections were viewed as slanted in Putin's favor.
Russian investigators suggested they may launch a probe into Gudkov's activities. A former intelligence official who took part in anti-Putin rallies last year, he is formally charged with operating a business while holding public office.
He's the second lawmaker to be expelled from the Duma without a court ruling since the Soviet Union collapsed in 1991.