A histological study of infection and patterns of development of the monokaryotic stage of Puccinia poarum on Tussilago farfara leaves has been made in the presence and absence of the carboxylic acid anilide fungicide, oxycarboxin. Penetration of epidermal cells by basidiospores was direct, and such penetrated cells developed a mycelial ‘coil’ or ‘aggregation’ from which inter- and intracellular hyphae emerged. Early in infection, high proportions of penetrated cells showed intracellular hyphae in close proximity to enlarged host nuclei which contained enlarged nucleoli. Later in infection, nuclei in the centre of the pustule began to degenerate and nuclei of penetrated cells at the margin of the colony showed less association with intracellular hyphae. Three main patterns or types of development were observed depending on the timing of dikaryotization. Type 1 had a short period of pycnial formation and sporulation, followed by immediate dikaryotization, giving rapid aecial formation and sporulation; Type 2, a longer period of pycnial production and sporulation then, after dikaryotization, rapid aecial maturity from preformed protoaecia; Type 3, only pycnial production and sporulation, and protoaecial formation with no dikaryotization. Irrespective of developmental type, mean colony growth remained linear. Treatment of infected leaves with 800 μg/g oxycarboxin decreased mean growth, and prevented or delayed maturation of the pustules of P. poarum. However, the effect was variable; some pustules were completely killed while others were apparently little changed. Possible reasons for such effects are discussed.