ABSTRACT Two heptapeptides with broad antifungal activity were identified and assessed for their ability to act synergistically with thiabendazole. The hexapeptide 66-10 was the progenitor of the heptapeptides and exhibited minimal inhibitory concentrations (MICs) of 9.3 to 9.8 mug/ml for thiabendazole (TBZ) resistant Fusarium sambucinum strains (MIC of 186 to 312 mug/ml). Heptapeptide derivatives 77-3 and 77-12 exhibited MICs between 3.8 and 7.5 mug/ml against the same strains. Incubation of conidia or mycelia with the peptide 77-3 showed that treated fungal structures were stained by the membrane impermeant dye SYTOX Green indicating disruption of membranes. Conidia incubated with peptide 77-3 at 10 mug/ml showed a 91 +/- 3.6% reduction in viability in 15 min. A checkerboard method was used to test the peptides and TBZ individually and in combination to determine potential synergistic activity. The results indicate that small peptides can act synergistically with TBZ against TBZ-resistant F. sambucinum.