Pseudoglyceryl ammonium ion coliposomes constructed from an arachidoyl probe lipid and either stearoyl or palmitoyl host lipids display unusually slow transverse bilayer migration ('flip-flop') of the arachidoyl probe molecule compared with the behavior of shorter probe lipids in similar colipsomes. This phenomenon is probably associated with interdigitation of the arachidoyl lipid across the bilayer's midplane.