Newport Beach-based Alliant Insurance Services Inc. has acquired the 401(k) arm of New York-based T&H Benefits, Thesco Retirement Planning Services.

Terms were not disclosed.

Alliant Chief Executive Tom Corbett said the acquisition of Thesco Retirement fits Alliant’s plans to increase the company’s presence in the 401(k) marketplace in the Northeast.

Alliant is the largest insurance broker based in Orange County, with revenue of about $81 million for the 12 months ended June, 2011.

The company has 21 property and casualty producers and six benefits producers in its Orange County office.

Alliant also provides workers’ compensation, employee benefits, surety, and financial products and services to about 20,000 clients nationwide.

It serves public entities, tribal nations, and the energy, healthcare, legal and construction industries, among others.