The Securities and Exchange Commission has requested data from mutual fund firms about how and why they pay to be included in 401(k) plans.
Fidelity Investments, Putnam Investments and T. Rowe Price Group confirmed that they recently received an inquiry from the SEC about practices related to so-called defined-contribution retirement plans.
The plan providers all said they were cooperating with the agency.
Mutual fund firms pay to be included in the fund lineups offered by companies such as Vanguard Group and Fidelity, which do a big business providing 401(k) administration to employers.
"We want to better understand the nature and purpose of these payments, including whether they are reimbursements for plan expenses or payments for shelf space or some other purpose," said Lori Richards, director of the SEC's office of compliance inspections and examinations, in a statement.
The SEC has previously confirmed that it's looking at why certain investments are recommended for 401(k) plans and pensions.
According to McHenry Group, an industry consulting group in Emeryville, Calif., the SEC sent a list of 25 questions to mutual fund firms over the last few weeks about so-called revenue-sharing arrangements in company-sponsored retirement plans.
The SEC letter asks how these arrangements are disclosed and whether certain funds receive "different positioning" as a result, according to McHenry Group.
The revenue-sharing arrangements aren't illegal, but critics say they can artificially inflate fees.