Aim: To identify antimicrobial peptides with high lytic activity against Rhizoctonia solani strain LR172, causal agent of rice sheath blight and aerial blight of soyabeans in the US. Methods and Results: Among 12 natural and synthetic antimicrobial peptides tested in vitro, the wheat-seed peptide, purothionin, showed the strongest inhibitory activity that was similar to the antifungal antibiotics, nystatin and nikkomycin Z. Cecropin B, a natural peptide from cecropia moth, and synthetic peptide D4E1 produced the highest inhibitory activity against R. solani among linear peptides. Membrane permeabilization levels strongly correlated with antifungal activity of the peptides. Noticeable changes in membrane integrity were observed at concentrations of greater than or equal to0.5 mumol l(-1) for purothionin, 2 mumol l(-1) for cecropin B, D4E1, D2A21, melittin, and phor21, and 8 mumol l(-1) for magainin II and phor14. An increase of nuclear membrane permeabilization was observed in fungal cells treated with cecropin B, but not with purothionin. Diffusion of nuclear content was observed by fluorescent microscopy 10 min after adding a lethal concentration of cecropin B. Evaluation by electron microscopy confirmed severe cytoplasmic degradation and plasma membrane vesiculation. Purothionin and cecropin B were the most stable against proteolytic degradation when added to liquid cultures of R. solani. Conclusions: Purothionin, cecropin B, D4E1 and phor21 were shown to exhibit high in vitro lytic activity against R. solani strain LR172 for rice and soyabean. These peptides are greater than 16 amino acids long and rapidly increase fungal membrane permeabilization. Resistance to proteolysis is important for sufficient antifungal activity of antimicrobial peptides. Significance and Impact of the Study: Selected antimicrobial peptides offer an attractive alternative to traditional chemicals that could be utilized in molecular breeding to develop crops resistant to rice sheath blight and aerial blight of soyabean.