Alfalfa and alfalfa weevil, Hypera postica (Gyllenhal), populations were treated with the fungicide chlorthalonil to eliminate the fungal pathogen, Erynia phytonomi , of the alfalfa weevil for comparison with untreated populations infected with the pathogen. Population curves for both populations were quite similar; however, the populations with a high incidence of the pathogen exhibited a more dramatic decline. The pathogen did not prevent weevil populations from attaining peak numbers and therefore provided little protection from crop damage.