Massachusetts employers will see a $150 million decrease in workers’ compensation insurance rates beginning July 1 under an agreement between state regulators and the insurance industry.

Attorney General Maura Healey, the State Rating Bureau and the Workers’ Compensation Rating and Inspection Bureau, which represents insurers, agreed this week to a 12.9 percent rate decrease for policies written after July 1. Insurance companies had initially filed for an 11.1 percent rate reduction.

Massachusetts employers must purchase workers’ compensation insurance, and rates are re-set at least every other year.

“This is certainly good news for employers. The 1991 workers compensation reform led by Associated Industries of Massachusetts is still paying dividends,” said Richard C. Lord, President and Chief Executive Officer of AIM.

Workers’ compensation rates have declined more than 60 percent since the 1991 reform.

Healey said in a statement, “When we lower the rates for workers’ compensation insurance, we protect workers and allow businesses to invest in higher wages and growth.”

AIM sponsors A.I.M. Mutual Insurance Company, one of the largest writers of workers compensation coverage in Massachusetts. Michael Standing, Chief Executive Officer of A.I.M. Mutual, thanked state regulators for an efficient rate-setting process.