Kicking off the second week of an antitrust trial, Neal Mohan testified that Google did what was best for consumers and the ad industry.

Neal Mohan, chief executive of YouTube, testified in federal court on Monday that Google acted in the best interest of consumers and the ad industry, refuting accusations by the Justice Department that the company crushed rivals as it became an advertising technology behemoth.

The YouTube chief’s testimony kicked off the second week of a trial over the Justice Department’s allegations that Google has a monopoly on the advertising technology market. Mr. Mohan, who joined Google as part of its acquisition of advertising software company DoubleClick in 2008, said that the search giant faced ample competition in the ad tech market. Google expanded into various areas of ad technology in response to demands from its business customers, including publishers and advertisers, he said.

Google’s success “falls back to one single thing: product innovation and the sale and services we were able to offer,” Mr. Mohan said, in his testimony before the U.S. District Court for the Eastern District of Virginia.

The case stems from a lawsuit, filed last year by the Justice Department and eight states, accusing Google of abusing control of its ad technology and violating antitrust law, in part through the acquisition of DoubleClick. Google has pushed up ad prices and also harmed publishers by taking a big cut of each sale, the government argues.

It is the second federal antitrust trial that the Silicon Valley company has faced in a year, with a federal judge ruling in the other case in August that Google had illegally maintained a monopoly over online search. Now that judge must decide how to fix the company’s search business.

The lawsuits are part of a greater push by regulators to rein in the power of technology companies, which control much of public discourse, commerce, and entertainment online. The Justice Department has also brought an antitrust suit against Apple, while the Federal Trade Commission has sued both Amazon and Meta, which owns Instagram and WhatsApp, over anticompetitive behavior.

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