After a 38-year debate on how to make trading costs for corporate and municipal debt transparent, regulators are making another attempt at forcing dealers to disclose how much they earn on the transactions.

The Municipal Securities Rulemaking Board will discuss a proposal at the end of the month, Executive Director Lynnette Kelly said yesterday, after U.S. Securities and Exchange Commission (SEC) Chair Mary Jo White asked the regulator to come up with a plan by year-end. The new rules would apply to so-called riskless trades, where firms fill client orders rather than use their own money to opportunistically buy.

Regulators are placing a greater emphasis on making sure smaller buyers don't get fleeced when transacting in the corporate- and municipal-bond market that's grown 36 percent since 2008. While stock brokers must tell investors how much they earn, bond dealers have profited from an opaque market where trades are still often completed over the telephone.