GILLETTE, Wyo. (AP) — Alpha Natural Resources is laying off 37 workers at its Belle Ayr and Eagle Butte mines in northeast Wyoming.

The layoffs announced on Thursday represent about 6.5 percent of Alpha's 563 workers at the two mines.

Low prices and weak market conditions have led Powder River Basin mines to lay off more than 500 workers since January in Wyoming.

The Mine Safety and Health Administration reports that at the end of the first quarter of 2016, Belle Ayr employed 271 workers, while Eagle Butte had 292. It wasn't made clear how many of the overall 37 layoffs came from each mine.

Bristol, Virginia-based Alpha Natural Resources filed for Chapter 11 bankruptcy reorganization in August.