The cell wall of the oomycete plant pathogen Phytophthora parasitica var. nicotianae contains a protein called CBEL that shows cellulose-binding (CB), elicitor (E) of defense in plants and lectin-like (L) activities. The biological role of this molecule in Phytophthora was investigated by generating transgenic strains suppressed in CBEL expression. Phenotypic characterization of these strains showed that they were severely impaired in adhesion to a cellophane membrane, differentiation of lobed structures in contact with cellophane, and formation of branched aggregating hyphae on cellophane and on flax cellulose fibres. Infection assays revealed that the strains suppressed in CBEL expression were not greatly affected in pathogenicity and formed branched aggregating hyphae in contact with the roots of the host plant, thereby indicating that CBEL is involved in the perception of cellulose rather than in the morphogenesis of hyphal aggregates. Interestingly, the absence of CBEL was correlated with abnormal formation of papillae-like cell wall thickenings in vitro, suggesting that CBEL is involved in cell wall deposition in Phytophthora. Reverse genetics in oomycetes has long been hampered by their diploid nature and difficulties in transformation and regeneration. The gene inactivation approach reported in this work provides the first direct evidence for intrinsic functions of an elicitor and cell wall protein in oomycetes.