Two strains, flocculating and nonflocculating, of the yeast Kluyveromyces lactis were grown in a Sabouraud's liquid medium containing 0.07 mM Ca. Treatment by ethylenediamine of isolated cell walls yielded three fractions: A, B, and C. Fraction A, soluble in ethylenediamine, contained phosphopeptidomannan-like hydrosoluble polymers; these constituted the external wall of the parietal layer. Phosphopeptidomannans have also been extracted from entire yeast cells autoclaved at 140 °C in a citrate buffer at pH 7.0. The flocculating and nonflocculating states of yeasts showed structural and quantitative variations in phosphopeptidomannans. The walls of the flocculating yeast cells contained higher amounts of peripheral polymers, the molecular masses of which were greater than those of nonflocculating yeast cells. These are the result of a more complex structure, due to the presence of a greater number of ramifications containing three or four mannose units. Analysis of the acetolysis products revealed in fact the release esentially of phosphorylated mannotriose and mannotetraose units by the flocculating yeast phosphopeptidomannans, while polymers of the nonflocculating yeast were characterized by the presence of mannobiose units. When such polymers were submitted to a β-elimination reaction, mannobiose and mannose units were liberated in such a ratio that mannobiose units appeared to be more numerous in flocculating yeast cells. A lectin extracted from the flocculating strain was incubated with erythrocytes activated by phophopeptidomannans derived from flocculating and nonflocculating yeasts and showed clearly that the more agglutinated erythrocytes were those activated by polymers derived from the flocculating yeast. The C fraction, insoluble in ethylenediamine, corresponded to the wall rigid matrix. The study of its chemical composition revealed no significative difference between the flocculating and the nonflocculating strains.Key words: Kluyveromyces lactis, phosphopeptidomannans, flocculation.[Journal translation]not available