Resistance against sunflower rust incited by Puccinia helianthi Schwein was characterized in three sunflower (Helianthus annuus L.) genotypes: AMES 18925 that was highly resistant; AMES 3442, partially resistant; and Hysun 33, susceptible. Microscopic studies showed a differential rust development on the genotypes. Impairment of rust spore germination and of appressorium formation was observed only in the resistant genotype. A differential excretion of coumarins to the leaf surface was also observed among genotypes, with the highest levels corresponding to the most resistant genotype and the lowest to the most susceptible. Impairment of rust germ tube growth and of appressorium formation was associated with the excreted level of ayapin. Reduction in germination frequency was not associated with the coumarins but probably to other phenolic compounds present at higher concentrations in the resistant than in the susceptible genotype.