From its origins in the South and Central American tropics, Parthenium hysterophorus has invaded locations in Australia, Asia and Africa, often with devastating impacts on health and livelihoods. Following studies on the distribution of parthenium in Ethiopia and southern Africa and subsequent recommendations for regional surveillance, this project investigated the actual extent of invasion by the weed to provide baseline data for its management in Kenya, Tanzania and Uganda. Roadside surveys undertaken on the basis of the CLIMEX simulation recorded parthenium in drainage trenches, dumpsites, abandoned buildings, construction sites, residential areas, rangelands and crop fields, thus emphasising the fact that successive dispersal and establishment of the weed closely follows patterns of disturbance. Results from this survey reiterate the need for immediate control of infestation in critical ecosystems and development of effective policy guidelines and strategies for management of this invasive species.