Updated at 3:30 p.m. PDT with additional detail and to correct the filing date for TomTom's suit, which was Monday.
TomTom has responded to Microsoft's patent suit by filing a patent claim of its own against the software maker.
The GPS device maker, based in The Netherlands, filed the countersuit in the U.S. District Court for the Eastern District of Virginia on Monday. TomTom says Microsoft's Streets and Trips products infringe on four patents it owns related to vehicle navigation software.
In a statement, Microsoft lawyer Horacio Gutierrez said the company is still in the process of reviewing TomTom's court filing.
"As has been the case for more than a year, we remain committed to a licensing solution, although we will continue to press ahead with the complaints we initiated in the U.S. District Court for the Western District of Washington and the International Trade Commission."
Microsoft sued TomTom in late February, accusing the company of infringing on eight patents. The suit was notable because it included, for the first time in a court filing, Microsoft's long-asserted position that the Linux kernel infringes on Microsoft patents.