We have investigated the temperature-concentration dependent phase diagram of an aqueous maltoheptaose-containing lipid, 1,3-di-O-dodecyl-2-(beta-maltoheptaosyl)glycerol Mal(7)(C-12)(2.) In the region examined (25-90 degrees C and 0-65 wt % lipid), two liquid crystalline phases form: a normal hexagonal phase (Hr) above 40-45 wt % lipid and a second phase (most probably a lamellar phase L-alpha) at higher temperatures (>55 degrees C) and higher concentrations (>60 wt % lipid). From the dilute solution to about 30 wt % lipid, an optically isotropic solution region is found where normal micelles exist. The present results together with the phase behavior of Mal(N)(C-12)(2)/water systems with N = 1-5 (Hato, M.; Minamikawa, H. Langmuir 1996, 12, 1658) indicate that a preferred phase of the Mal(N)(C-12)(2) where the headgroup is composed of N glucose residues linked via beta-1,4O-glycosidic bonds shifts from an H-II to an H-I via an L-alpha phase as N increases. The observed phase sequence makes a marked contrast to the Cel(N)(C-12)(2)/water systems where the headgroup is composed of N-glucose residues linked via beta-1,4-O-glycosidic bonds (Hato, M.; Minamikawa, H. Langmuir 1996, 12, 1658). The stereochemistry-dependent phase behavior can be interpreted in terms of different conformations of the headgroups, i.e., a "helical" conformation of the maltooligosaccharide headgroups, and an "extended" conformation of the cellooligosaccharide headgroups.