Sonchus species (Sowthistles) have a world-wide distribution and are serious weeds of crops and the environment. We assessed the suitability of the rust, Miyagia pseudosphaeria, for use in biological control of Sonchus species in Australia and elsewhere. Testing of a limited range of plant species showed the host-range of the rust to be restricted to Sonchus species. We measured spore germination and development of the rust on the host plant in relation to temperature. These measurements, the biology and distribution of the rust, and that of the host S. oleraceus, were used to model the potential world distribution of the rust in CLIMEX. The model indicated that there was limited potential for further spread in Australia; the rust already being widespread. The rust is not present in Canada, but the CLIMEX model indicated that cold temperature will be the main limiting factor for the rust should it be introduced, compromising its suitability as a biological control agent. In South America the rust could potentially be introduced as a biological control agents in areas with Mediterranean and subtropical climate. Molecular studies in combination with more comprehensive inoculation and temperature studies are needed to establish if there are pathotypes of the rust that might be more suitable for use in biological control.