SummaryCommon heliotrope,Heliotropium europaeum(Boraginaceae), a summer annual, is a serious weed of pastures in Australia causing poisoning in sheep and other grazing animals. The weed is native to Mediterranean and Middle Eastern regions of Eurasia where it is attacked by several arthropods and pathogens including the rust fungusUromyces heliotropii.The host‐specificity of a virulent and effective strain of the rust from Turkey has been tested by inoculating 96 plants of importance to the Australasian region using both microscopic and macroscopic observations of the reaction of host and non‐host plants. The test plants included several members of the Boraginaceae and related families as well as species ofHeliotropiumnative to Australia. The infection was mostly limited to European species ofHeliotropium. U. heliotropiiwas thus found to represent a safe introduction and has recently been introduced to Australia for the biological control of common heliotr