In one of its strongest sales periods since 2005, Boston-based Fidelity Investments reported it has received new defined contribution commitments representing nearly 400,000 participants and $22 billion in assets under administration in the first half of 2010.

Fidelity, which provides employer benefits, signed plans this year with Fortune 500 company Halliburton, with 33,500 participants, and mid-size companies such as Southwire, Oldcastle, and NetSuite.

The company also reported strong sales in the tax-exempt market during the first six months of the year. Fidelity improved its No. 1 market position in the health care industry by signing on health care institutions such as Tufts Health Plan.

Fidelity provides benefit services to nearly 16,000 employers and 20,000 plans representing more than 20 million participants. Its benefit offerings include record-keeping of defined contribution, defined benefit, health and welfare, health savings accounts, stock plan services and benefits counseling.