Arenicin-1 (Ar-1) is a beta-sheeted antimicrobial peptide from the marine lugworm Arenicola marina. To elucidate the significance of its unique 18-residue cyclic structure and of six cationic arginines for its biological activity and its interaction with biomembranes, we synthesized one linear peptide in which the two cysteines were exchanged for serines (C/S-Ar-1) and a cyclic peptide in which all arginines were replaced by lysines (R/K-Ar-1). We addressed antibacterial and hemolytic activities, the impact of the peptides on bacterial morphology, and their binding to, intercalation into, and permeabilization of model membranes composed of phospholipids or lipopolysaccharide (LPS). In accordance with high salt concentration in sea water, the antibacterial activity of Ar-1 was almost insensitive to high NaCl concentrations. In contrast, the linear derivative lost activity under these conditions against polymyxin B-resistant Proteus mirabilis. Ar-1 intercalated into phospholipid and LPS membranes and formed heterogeneous and short-lived lesions. However, when the peptide was present in both membrane leaflets, it formed defined pores. This characteristic was not observed for the linear derivative C/S-Ar-1. Apparently, the disulfide bond provides conforma-tional stability, which has an impact on salt tolerance, prevents fast degradation by trypsin, and is a prerequisite for the formation of structurally defined pores.