The in vitro and in vivo variation of selected isolates of Phoma medicaginis var. medicaginis was studied in controlled environmental conditions. Isolates were relatively stable in morphology and pigmentation of the colony but varied more in final diameter of the colony, rate of growth and sporulation. The isolate, the organ and geographic location from which it was obtained were significant sources of variation. All isolates caused root and crown rot symptoms in growth chamber and greenhouse experiments. Differences in virulence among isolates were expressed in the length of root lesions. Plant dry weights were reduced by 82% in greenhouse trials.