By monitoring increases and decreases in the proportion of cycloheximide-resistant macroconidia, periodic selection was observed in populations of the filamentous fungus Fusarium graminearum, grown in glucose-limited chemostat cultures. The results indicated that periodic selection of advantageous mutants of F. graminearum occurred at intervals of about 124 h at both high (D = 0.19 h-1, approximately 34 generations) and low (D = 0.06 h-1, approximately 11 generations) dilution rates. Several 'adaptive' peaks (each indicating the appearance of an advantageous mutation) were observed before morphological (highly branched) mutants appeared in the populations; these mutants have previously been observed to have a selective advantage over the parental strain. At intervals, macroconidia harvested from the chemostat were used to inoculate plates of non-antibiotic-containing agar medium, and it was possible to monitor periodic selection in the original chemostat culture using second generation macroconidia harvested from these cultures. The proportion of cycloheximide-, potassium chlorate-, and p-fluoro-DL-phenylalanine-resistant macroconidia in these second generation macroconidia changed in a pattern similar to that observed when monitoring the proportion of cycloheximide-resistant macroconidia in the first generation population harvested directly from the chemostat. The experiments demonstrated that populations of filamentous fungi are heterogeneous and that much of this heterogeneity may already be present at the end of batch growth, i.e., before the onset of continuous cultivation.