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.