The isotropic (I)-cubic (Q(223)) phase transition is studied in micellar aqueous solutions of palmitoyllysophosphatidylcholine (PLPC), using X-ray scattering. A mechanism to describe the I-Q(223) phase transition is proposed in terms of the local order in the concentrated I phase and of the long-range order in the lower concentration limit of the Q(223) phase. Information on the micellar shape in the concentrated I phase, previously obtained by us, and in the Q(223) phase, now obtained, together with previous NMR and X-ray diffraction results of the literature, are used in our analysis. It is found that a local cubic order already exists in the concentrated I phase, but the positional correlation is between clusters with 5 micelles. The measured distance between these clusters is equal to the distance between micelles centered in one of the special positions of the cubic structure of phase Q(223) (which has 8 micelles in the unit cell with 2 special positions). The total micellar volume in the Q(223) phase is compatible with micelles of same paraffinic volume as those already studied in the I phase.