PubMatic, a programmatic ad selling platform, is acquiring mobile ad-server Mocean Mobile (formerly Mojiva) in a bid to strengthen its ability to sell ads on phones, the company announced today.

The acquisition price was $15.5 million dollars according to a source familiar with the deal, making it PubMatic's largest acquisition to date.

The deal comes on the heels of demand side platform MediaMath's acquisition of cross-device targeting company TactAds. Simplytics, another mobile ad server, was snatched up by Integral Ad Science earlier this year. The deal also follows a partnership PubMatic announced last week with mobile ad-tech company xAd, which it will work with to bolster its location targeting offering.

In an interview, PubMatic president Kirk McDonald said the acquisition is aimed at strengthening PubMatic's ability to win a publishers' entire businesses, not just desktop.

"For many publishers, I have kept mobile separate from what I was doing in display," he said, speaking from the publisher perspective. "We have to stop segregating mobile."

Many of today's dominant ad-tech companies built their businesses by automating desktop ad transactions, focusing their attention where the vast majority of digital ads appeared. But with consumer attention rapidly migrating to mobile -- where these companies' tech doesn't work as well -- they're making a run on mobile ad-tech firms to keep up.

The deal, Mr. McDonald said, "really starts the promise of a simplified, one-platform offering where publishers no longer have to rely on multiple platforms." PubMatic has been in market with a mobile offering for 18 months but without a mobile ad-server.

Besides for eliminating PubMatic's need to integrate with third party ad-servers, the acquisition should also help it do a better job identifying the value of mobile impressions to its publisher clients, enabling them to sell clusters of cross-device audience more effectively.

Mocean Mobile, founded in 2008, has 40 employees. This marks a disappointing end for the company, which raised over $45 million dollars.