The stability of the so-called ''gyroid'' phase, of <Ia(3)over bar d>
symmetry, observed for lyotropic liquid crystals and block copolymers
is accounted for using the Landau-Brazovskii theory of weak crystalliz
ation. In the mean field approximation, the <Ia(3)over bar d> phase is
stable between the lamellar and hexagonal phases close to the disorde
red liquid phase. When allowance is made for Brazovskii critical fluct
uations, a direct phase transition between the disordered liquid and <
Ia(3)over bar d> phases is shown to be possible, in agreement with exp
erimental observations. (C) 1996 American Institute of Physics.