ABSTRACT Parthenium is an exotic invasive weed that originated in South America now causing severe infestation in Ethiopia. This weed is known to be infected by various micro organisms such as the rust pathogen that may be used as a component of integrated weed management. In Ethiopia epidemiology of this pathogen is not well studied. Field surveys were conducted in 2008 and 2009 cropping seasons in eastern Ethiopia to study the epidemiology of the rust (Puccinia abrupta var partheniicola) on this weed. In addition the effect of this pathogen on morphological parameters of parthenium was studied both in the field and in the greenhouse at Plant Protection Research Center, Ambo. A total of 218 fields were surveyed in four districts (Chiro, Gemechis, Kuni and Tullo) of Western Hararghe Zone, eastern Ethiopia. All the fields surveyed were infected with the pathogen indicating the disease was widely distributed in the zone. The incidence of rust in the surveyed area was in the range of 25 to 74% while severity was in the range of 18 to 55%.