The close proximity of established colonies of Uromyces phaseoli var. typica (the bean rust fungus), U. phaseoli var. vignae (the cowpea rust fungus) and Puccinia helianthi (the sunflower rust fungus) in their respective hosts increased the number of infection sites at which incompatible rust fungi produced haustoria from first-formed haustorial mother cells. Injection of extracts from susceptible rust-infected leaves of French bean [Phaseolus vulgaris ''Pinto''] had no effect on subsequent haustorium formation by the sunflower rust fungus in bean or cowpea [Vigna sinensis ''Early Ramshorn''] or by the cowpea rust fungus in sunflower [Helianthus annuus ''Sunrise'']. These extracts did increase haustorium production by the bean rust fungus in cowpea and by the cowpea rust fungus in bean; the effective component was of low MW and partially heat labile. In both double-inoculation and injection experiments, only in the cowpea rust fungus-bean interaction did fungal growth continue beyond the formation of the 1st haustorium; such growth also was elicited by extracts from susceptible rust-infected cowpea leaves and by the close proximity of already growing colonies of the cowpea rust fungus induced to develop in bean leaves by a preinoculation heat shock of the nonhost tissue. It is suggested that in susceptible rusted French bean and cowpea leaves, and in interactions of French bean with the cowpea rust fungus if fungal colonies are permitted to develop by giving the tissue a heat shock, a similar or identical factor (or factors) is produced which, in French bean plants, specifically inhibits the expression of some, but not all, defense mechanisms which can prevent incompatible rust fungi from forming haustoria. Growth of rust fungi in any plant beyond the formation of the 1st haustorium seems to depend on additional interactions between the 2 organisms. Significant differences in the response of different cultivars of French bean towards the cowpea rust fungus also were observed.