Selection occurring during the saprophytic or overwintering phase of the life cycle of Cochliobolus heterostrophus, the causal agent of southern leaf blight of maize, may be a factor in the persistence of apparently less aggressive isolates in the pathogen population. The relative aggressiveness and ability to perennate of 22 isolates of C. heterostrophus from North Carolina was measured in series of experiments. Significant differences in aggressiveness and percent perennation (overwintering survival) were observed. There was a weak but often significant negative correlation between the ability of isolates to persist on the soil surface and their aggressiveness. The ability of race O isolates to sporulate on senescent corn leaf discs was positively correlated with their aggressiveness. Selection against increased aggressiveness during overwintering does not appear sufficient by itself to counter selection for increased aggressiveness occurring during the pathogen's pathogenic phase.