Standard & Poor's Ratings Services said it downgraded the debt of 165 U.S. companies in the fourth quarter of 2015, representing $1.5 trillion, led by the oil, gas, and commodity sectors.
For the full year, downgrades -- meaning debt is at greater risk of default -- rose by 67 percent, to 461, S&P said, the highest level since 2009.
Plunging oil and commodity prices contributed to the downgrades in energy, metals, mining, and steel sectors, the ratings company said. Those sectors accounted for one-third of the downgrades in the fourth quarter.
Financial institutions accounted for 73 percent of debt downgrades in the quarter, as S&P said it took into account the US government backing away from "extraordinary" support of the sector.
US stocks rebounded Thursday but have been battered so far this year by falling oil and commodity prices.
Beth Healy can be reached at beth.healy@globe.com. Follow her on Twitter @HealyBeth.