The effect of sarcotoxin IIA, an antibacterial protein of Sarcophaga peregrina (flesh fly), on Escherichia coli was investigated. Sarcotoxin IIA was found to have a bacterial effect on growing bacteria, but little on non-growing bacteria. At a concentration of 25 micrograms/ml, it induced significant morphological change of growing E. coli cells. In its presence, growing cells became greatly elongated, and spheroplast-like bulges and projections appeared on their surface. A rough mutant strain of E. coli with a defect in the structure of lipopolysaccharide was more sensitive than the parent strain to sarcotoxin IIA. These results suggest that the main effect of sarcotoxin IIA is to inhibit cell wall synthesis, including septum formation.