In the letter to the FTC, San Mateo, California-based startup Certree said that Equifax and Experian are providing financial incentives like a slice of their revenue to employers to gain exclusive access to payroll data.
Equifax also has deals with payroll software vendors that help employers process paychecks.
Equifax in March described its Workforce Solutions unit, which included the payroll data on 105 million people, as its "fastest growing, highest margin, and most valuable business," with just over $2 billion in revenue.
The company said workers can view or block access to records and that it has improved data security.
But in the letter, Certree said the exclusivity deals pose "enormous obstacles."