The interaction of the voltage-dependent channel-forming peptide alamethicin with dioleoylphosphatidylcholine (DOPC) small unilamellar vesicles (SUV) has been studied using circular dichroism spectroscopy over a range of wavelengths and temperatures. Evidence is presented for the existence of two distinct membrane-bound states of the peptide which reflect different extents of peptide-peptide interaction. An elevated temperature is found to diminish the apparent peptide-peptide interaction. These results provide insight into the general problem of helix-helix interaction in membranes and provide experimental support for the proposal [Popot, J. L., & Engelman, D. M. (1990) Biochemistry 29, 4031-4037] that these interactions can be enthalpically favorable.