Addition of sulphoquinivosyldiacylglycerol (SL) to mixtures of monogalactosyldiacylglycerol (MG) and digalactosyldiacylglycerol (DG) induces the appearance of paracrystalline arrays of 80–100 Å lipidic particles. The hypothesis is presented that SL occupies the position in the leaflet opposite the micelle of MG or the position in the leaflet on the convex side of the bulge or cusp in the corresponding model for lipidic particles, and that these orderly arranged lipidic particles containing negatively charged SL may form part of the basis for the orderly arrangement of other molecules functioning in photosyntesis in the thylakoid membrane.