In order to study the secondary structural preferences of amphipathic cyclopeptides in detergent assemblies and their interactions with metal ions, two basic amphiphatic cyclooctapeptides, c[(Lys- d-Lys) 2-Xaa- d-Leu-Leu- d-Leu], where Xaa is Leu ( P1) or Trp ( P2), were studied by circular dichroism (CD) spectroscopy. In water, P1 exhibited an unordered secondary structure whereas P2 adopted a partial β-sheet structure. Temperature effect on the CD of these cyclopeptides showed small changes over the temperature range from 278 to 353 K. P1 showed low and non-specific binding affinity for metal ions (Ca 2+, Zn 2+, Na +, or Li +) in water whereas P2 did not exhibit any significant interaction with these ions. However, in the zwitterionic micellar detergent dodecylphosphocholine (DPC), P2 adopted a β-sheet structure, which exhibited a greater propensity for metal ion interactions (K 1∼10 3 M −1 and K 2∼10 2 M −1). Variable temperature CD studies on the peptide–metal ion complexes showed that these interactions are thermally reversible. Our results indicate that amphipathic cyclooctapeptides can co-assemble with micellar DPC and are capable of interacting with metal ions. This study will improve our ability to design a better metal ion sensing cyclopeptide in co-micellar assembles.