The wheat stem sawfly, Cephus cinctus Norton, is a widely distributed and destructive key pest of wheat production in the northern Great Plains of the United States. The larvae overwinter in diapause within cut stems in the wheat residue at or below the soil surface for 8 to 9 months. Sawfly larvae in the cut stems of the wheat residue are vulnerable to plant pathogens infecting wheat as well as to soil microorganisms during the long overwintering period. Previous field observations by the authors have suggested that there were dead and diseased overwintering sawfly larvae in the wheat residue. Fusarium species were isolated from the diseased sawfly larvae, including F. acuminatum, F. equiseti, F. avenaceum, F. culmorum, and F. pseudograminearum. The current research was a survey of sawfly infested stems in a Montana wheat field to determine the postharvest mortality caused by Fusarium species and other fungal organisms. The percentage of fungal colonized wheat stems varied from 5.73% to 36.96% at Churchill, MT in April 2006. The mortality of the overwintering sawfly larvae was 94.21% in the fungal colonized sawfly-cut stems. Postharvest mortality by Fusarium species on the overwintering sawfly larvae was conducted both in greenhouse and field trials on the MSU Post Farm, sawfly larvae mortality ranged from 23.86% to 44.97% in the greenhouse and from 40.31% to 60.18% in the field trial. All the tested Fusarium species, including F. pseudograminearum, F. culmorum and F. acuminatum, showed statistically significant suppression of the overwintering sawfly larvae relative to the control.