Comparison of eriophyid mites occurring on Cirsium arvense (L.) over two growing seasons showed two forms of females which were previously called separate species Aceria? anthocoptes (Nalepa, 1892) and Aceria leonthodontis (Lindroth, 1904). According to our data, two forms of females belong to one species Aceria anthocoptes (Nal.). Males studied during the whole growing season had the characteristics similar to protogynes. Damages caused by both forms were the same. In the middle of the season transitional forms between protogynes and deutogynes were often found.