Russia has ordered a Latvian diplomat to leave the country - a move seen as retaliation for Latvia's expulsion of a Russian diplomat earlier this week.
The unnamed Latvian diplomat must leave Russia within two days, Russia's foreign ministry said.
It said the official had been expelled "for activities incompatible with the status of a diplomat" and "damaging for Russia's security interests".
On Monday, Latvia announced its expulsion of the Russian diplomat.
The foreign affairs ministry in Riga said he diplomat posed a security threat to Latvia and had 48 hours to leave the country.
The two former Soviet states had made strides to improve ties over the past few months, after years of strained relations.
Riga and Moscow signed a border treaty in March 2007, making the boundary between the countries official for the first time since the collapse of the Soviet Union in 1991.
However, Russia continues to accuse Latvia - now an EU and Nato member - of discriminating against its large ethnic minority. Riga denies the charge.