Extrafloral nectaries of Cassia fasciculata attract nectar feeding ants which protect the plant against leaf herbivores. High ant visitation in late July coincided with high herbivore densities at two sites in east central Iowa. The highest level of leaf herbivory occurred during the time of flowering and early fruit filling, just after the peak of herbivore and ant activity. Results of ant exclusion experiments at the two sites showed that ant visitation resulted in decreased herbivore numbers, decreased leaf area loss, increased growth, and at one site decreased plant mortality. However, this reduction in leaf area loss and increase in growth did not translate into seed set differences between plants with and without ants at either site. Initial plant size was more important than the presence or absence of ants in determining fecundity for this temperate annual during a year of summer drought.