A 15-residue hybrid peptide containing residues 1–7 from cecropin A and residues 2–9 from melittin, CA-(1–7)M(2–9), is a potent antibiotic with broader activity than cecropin A, but without the cytotoxic character of melittin. The conformational behaviour of CA(1–7)M(2–9) including the formation of multimeric species in solution has been investigated by circular dichroism, ultracentrifugation, electrospray mass spectrometry, NMR and energy calculations. Addition of hexafluoroisopropanol or liposomes causes the appearance of a CD spectrum characteristic of a helical structure that changes with pH, buffer and peptide concentration. The concentration dependence is atypical, as the ellipticity at 222 nm decreases with peptide concentration and is not correlated with a corresponding decrease in helix content as measured from the NMR spectra. The presence of aggregated structures is demonstrated by ultracentrifugation and ES-MS experiments, which also provide an indication of the stoichiometry. Longrange NOEs suggest a model of aggregation with neighbouring molecules packed antiparallel. Aggregation causes very slow proton-deuterium exchange in some amide protons in the C-terminal region and provides a method for estimating a very large association constant (ca. 106M−1) as well as the stoichiometry of the aggregates. The tendency to aggregate seems to be an inherited feature from melittin and may enhance the antibiotic activity either by facilitating the incorporation of the peptide into the membrane in large quantities or by promoting the disruption of the membrane.