Two types of abnormal plants have been observed in clonal plantations of perennial ryegrass. Plants of the first group were characterized in early spring by a dense dwarfed growth of distorted blue-green leaves. Those of the second group became recognizable later in the season, because of their somewhat stunted growth and the yellow, orange or pale-red discoloration of the upper halves of the leaves. No consistent visible pathogen could be detected but when aphids, fed on the abnormal grass plants, were transferred to seedlings of oats and barley, these reproduced typical symptoms of the cereal yellow-dwarf disease. The abnormal ryegrass plants of both groups may contain the same virus. It is suggested that the first group represents plants infected in the preceding year, the last group those infected in the current season. Whether the virus is alone responsible for the damage, still remains to be ascertained.