The federal Investigative Committee said December 14 that Navalny and his brother Oleg were being investigated over the alleged theft of $1.79 million by a trading company with which they are involved.
Navalny, an anticorruption blogger who has organized protests over the past year against President Vladimir Putin, already faces charges of theft that he says are politically motivated.
The Investigative Committee's announcement comes a day before the opposition is due to stage an anti-Putin march in Moscow.
Navalny reacted to the announcement by saying on Twitter: "I'm not enough for them, they are now after my family."
He said that the apartments of his brother and his parents were being searched.
Navalny faces up to 10 years in jail if found guilty of the earlier charges.