On the basis of light microscopic (LM), scanning electron microscopic (SEM) and transmission electron microscopic (TEM) investigations the <jats:italic>Pertusaria</jats:italic>-type of ascus is described as a particular functional type. The functionally unitunicate <jats:italic>Pertusaria</jats:italic>-type is characterized by its structure, staining properties, and by its particular mode of dehiscence. Tripartite ascus walls were observed in LM and TEM. The non-amyloid ascus wall is surrounded by a thin, amyloid outer layer. Both become amorphous at maturity and partly disintegrate. An apically thickened, amyloid inner layer reaches the base of the ascus. In its fine structure this amyloid inner layer resembles the material of the amyloid dome of <jats:italic>Lecanora</jats:italic>-type asci. It plays an important role during dehiscence and spore discharge. An elongation process was observed prior to dehiscence, at the end of which the ascus tip is situated above the hymenial surface. Dehiscence occurs by bursting or splitting of the whole ascus tip. The <jats:italic>Pertusaria</jats:italic>-type might represent a side-branch of evolution from bitunicate to unitunicate forms within the Lecanorales.<jats:italic>Pertusaria</jats:italic>-type asci are restricted to a small number of genera within the Pertusariaceae. A considerable heterogeneity in ascus structure and staining properties was observed within the Pertusariineae sensu Henssen &amp; Jahns (1973) and Henssen (1976).