Differences in pathogenicity of various Ophiobolus graminis isolates have been observed frequently (Davis 1925; Padwick 1936; Hynes 1937; White and McIntyre 1943; Henry and Gilpatrick 1947; Henry and McKenzie 1959). However, most workers give little or no information about the interval between isolation and testing of pathogenicity, although it is evident some isolates have been in culture for long periods. As virulence may vary considerably with repeated subculturing (Russell 1934, 1939), it is possible that some studies have been made with cultural variants which do not exist in nature. This communication, therefore, describes tests using the initial growth from fresh monosporous field isolates of O. graminis.