Important morphogenetic transitions in fungi are triggered by starvation-induced changes in the expression of structural surface proteins. Here, we report that nutrient deprivation causes a strong and reversible up-regulation of TbSP1, a surface-associated, Ca 2+ -dependent phospholipase from the mycorrhizal fungus Tuber borchii. TbSP1 is the ®rst phospholipase A 2 to be described in fungi and identi®es a novel class of phospholipid-hydrolyzing enzymes. The TbSP1 phospholipase, which is synthesized initially as a preprotein, is processed ef®ciently and secreted during the mycelial phase. The mature protein, however, also localizes to the inner cell wall layer, close to the plasma membrane, in both free-living and symbiosisengaged hyphae. It thus appears that a dual localization phospholipase A 2 is involved in the adaptation of a symbiotic fungus to conditions of persistent nutritional limitation. Moreover, the fact that TbSP1-related sequences are present in Streptomyces and Neurospora, and not in wholly sequenced non-®lamen-tous microorganisms, points to a general role for TbSP1 phospholipases A 2 in the organization of multicellular ®lamentous structures in bacteria and fungi.