Various features of the biology of the rust fungi and of the epidemiology of the plant diseases they cause illustrate the important role of rainfall in their life history. Based on this insight we have characterized the ice nucleation activity of the aerially disseminated spores (urediospores) of this group of fungi. As this fungus cannot be cultivated, urediospores of this obligate plant parasite were collected from natural infections of weeds and cultivated plants in Brazil, France, the USA, Turkey and Syria. Spores in all samples carried bacteria and in most of the samples they carried ice nucleation-active strains of Pseudomonas syringae (at rates of less than 1 bacterial cell per 100 urediospores). Therefore, demonstration of the inherent ice nucleation activity of the urediospores was based on a series of experiments that allowed quantitative measures of the contribution of spores and bacteria to the total number of ice nuclei in mixtures of these two components. Rust spores had onset freezing temperatures as warm as -4°C and in all samples bacterial INA accounted for only a small fraction of the INA observed in spore suspensions. Changes in the INA of spore suspensions after treatment with lysozyme suggest that the INA of urediospores involves a polysaccharide. Based on data from the literature, the concentrations of urediospores in air at cloud height and in rainfall were estimated to be very similar to those reported for other biological ice nucleators. Under conditions of massive spore production during disease epidemics, the concentrations at cloud level might be very high due to convective air movements. How this phenomenon could lead to overseeding of clouds will be presented.