(1) Seedlings of groundsel (Senecio vulgaris L.) were planted into tubs of soil sunk to the level of the normal soil surface at a field site in late spring. Plants in half the tubs were inoculated with rust fungus (Puccinia lagenophorae Cooke). (2) Infection inhibited the expansion of leaves and reduced plant dry weight. The pattern of partitioning of dry weight between organs was little changed. Senescence occurred earlier and more rapidly in rusted plants than in controls. (3) Cumulative production of capitula was inhibited by 43% in infected plants; the number of florets per capitulum was also slightly lower in infected plants than in control plants, resulting in a 46% reduction in overall floret production. (4) Little seedling mortality occurred, but mortality of mature, flowering plants began earlier and was more rapid in inoculated populations than in controls. This greater mortality, combined with the reduced growth of infected individuals, resulted in substantially lower vegetative production in rusted populations. (5) Infection reduced the percentage of plants which flowered, and cumulative production of capitula was some 60% lower in infected than in healthy populations. Cumulative floret production was 64 000 m-2 in control populations, but only 25 000 m-2 in infected populations. (6) Responses of the summer population to rust are contrasted with the previously reported responses to rust of an over-wintering population.