Tibi grains consist of a unique and very stable symbiotic association of Lactobacillus brevis, Streptococcus lactis and Saccharomyces cerevisiae embedded in a dextran matrix. The structural organization of the grain was examined by light, scanning and transmission electron microscopy. The grain was constituted of an outer compact layer and an inner spongy structure. The outer layer was more densely populated by the microorganisms than the inner layer but dextran, stained on frozen thin sections with fluorescein-conjugated concanavalin A, was more abundant in the inner layer.