Zoom Video Communications late last week filed a trademark infringement case against rival RingCentral that highlights the shift in the relationship between the two companies from important partners to bitter rivals.

In the lawsuit, filed in the Northern District of California, Zoom (ticker: ZM) notes that RingCentral (RNG) entered a partnership with Zoom to resell its videoconferencing software in 2013. But Ring last April announced plans to begin shifting customers using its broader communications suite to its own video platform, and then debuted a free-standing version of its video software in December, directly competing with Zoom’s core business.

In a heavily redacted version of the complaint, Zoom asserts that RingCentral has continued to market and resell Zoom’s products to new customers “despite repeated requests from Zoom that it stop,” and that it continues to make use of Zoom’s trademarks to aid that effort.

In a statement, RingCentral said that it disputes Zoom’s allegations.

Zoom shares were down 1.6%, at $344.28, in recent trading, while Ring was off 3.8%, to $331.10. The S&P 500 was up 0.1%.

(Source: Barron’s)