The development of uredinia of Uromyces rumicis (Schum.) Wint. was studied on Rumex crispus L. and R. obtusifolius L. plants inoculated in the laboratory. Fewer uredinia developed on leaves injured by simulated insect herbivory, those fed upon by Gastrophysa viridula Degeer (Coleoptera: Chrysomelidae), and young, incompletely expanded leaves, than on uninjured, fully expanded control leaves. The effect of simulated herbivory and leaf age on the process of U. rumicis infection was investigated using Calcofluor staining and epifluorescence microscopy. Simulated insect herbivory reduced significantly the proportion of sporelings producing appressoria and the proportion of sporelings with appressoria that entered the substomatal cavity. In R. obtusifolius, leaf injury also reduced the proportion of penetration hyphae that formed substomatal vesicles. The proportion of sporelings that produced intercellular hyphae in injured leaves was reduced by 58 % in R. crispus and 89 % in R. obtusifolius. G. viridula feeding induced similar resistance in R. obtusifolius leaves. Simulated insect herbivory reduced both uredinial density and the area occupied by intercellular hyphae by between 65% and 89%. Injury had a greater effect than leaf age on sporelings in the pre-haustorial stages of development. However, the total area occupied by intercellular hyphae in young leaves was reduced by between 42% and 78% compared to mature leaves, owing to a lower uredinial density and, in R. obtusifolius, also to a significant reduction in the size of individual uredinia.