Abstract: Lactic acid bacteria (LAB) Lactobacillus plantarum ML11-11, an isolate from Fukuyama pot vinegar, and yeast Saccharomyces cerevisiae form significant mixed-species biofilm with direct cell-cell contact. Co-aggregation of L. plantarum ML11-11 and S. cerevisiae cells, mediated by the interaction between surface protein(s) on L. plantarum ML11-11 cells and surface mannan of S. cerevisiae cells, contributes significantly to mixed-species biofilm formation. In this study, co-aggregation activities of yeast mutants that were deleted of genes related to mannan biosynthesis were investigated to clarify the mannan structures essential for interaction with L. plantarum ML11-11. Among the 12 deletion mutants which had various incomplete mannan structures, only the mnn2 mutant lost the co-aggregation activity. In the mnn2 mutant, the gene coding the activity of attaching first branching mannose residue to mannan main chain is deleted and therefore the mnn2 mutant has unbranched mannan. From this result, it is clarified that the specific structure, consisted of mannan main chain to which are attached side chains containing one or more mannose residues, is critical for co-aggregation with L. plantarum ML11-11.