The wheat stem sawfly, Cephus cinctus Norton, is a widely-distributed and destructive pest of winter wheat production in the Northern Great Plains. The larvae overwinter in diapause within sealed, sawfly-cut wheat stems in postharvest stubble at or below the soil surface for 8 to 9 months. Previous field observations suggested that dead, overwintering sawfly larvae were caused by Fusarium species occurring in the wheat stubble. We surveyed the percentage of overwintering larval mortality in 5 locations in Montana in the spring of 2007. Larval mortality in the overwintering cut stems ranged from 10.0% to 20.0% across locations. Naturally-occurring Fusarium species were the dominant microorganisms isolated from larval sawfly cadavers, and isolated species were F. pseudograminearum, F. culmorum, F. acuminatum, F. avenaceum and F. equiseti. We used isolates to assess mortality caused by the Fusarium species on the actively-feeding, developing larvae inside stems of growing wheat plants during the summer. The experiment was conducted by inoculating winter wheat with selected Fusarium species at planting and infesting them using ovipositing sawfly adults at stem elongation stage. Larval mortality was quite high in the developing stems, with F. pseudograminearum, F. culmorum and F. acuminatum killing 78.7%, 70.2% and 40.7% of the growing larvae respectively, while control mortality was only 11.5%. Subsequent Fusarium toxin detection in the wheat stems and sawfly bioassay with deoxynivalenol toxin from Fusarium also supported the possibility of this toxin being a component in the larval mortality occurring in this system.