A Swiss man has been arrested in Frankfurt on suspicion of spying.
The 54-year-old, named only as Daniel M, was taken into custody on Friday, prosecutors said, but an arrest warrant had been issued for him in December.
German federal prosecutors said the man was suspected of having worked "for the intelligence service of a foreign power" since early 2012.
Several residential and business premises in Frankfurt and the nearby Wetterau region were searched by officers from the Federal Criminal Police Office (BKA).
Reports suggest the man may have spied on German tax investigators.
Tax authorities in Germany have controversially bought CDs of information from whistleblowers in Swiss banks as they try to catch German residents with Swiss bank accounts.
The issue has caused friction between the Swiss and German governments.
Authorities in Germany's federal states have justified the hefty payments by saying the information gained would lead to much larger sums in unpaid taxes being retrieved, but Switzerland thinks paying people who steal information from Swiss banks is wrong.