Ascus structure of eight yellow, two white and two brown Rhizocarpon species has been investigated by light microscopy. Ultrastructure and function in R. atroflavescens subsp. pulverulentum and R. montagnei were studied in TEM. The Rhizocarpon-type ascus clearly differs from all other ascus types observed in the Lecanorales. It is bitunicate, opening with a slight ‘Jack-in-the-box'-mechanism. Its structure and function are related to patellariacean ascus types, but unlike those the ascus wall cytochemistry shows a certain similarity with Lecanora- and Peltigera-type asci. Rhizocarpon-type asci are embedded in a strongly amyloid hymenial gelatine. The nonamyloid ascus wall is surrounded by the strongly amyloid outer layer. The slightly amyloid expansible inner layer (= endoascus) is apically thickened; it shows the banded and pleated ‘accordion-structure' characteristic of bitunicate asci. Prior to dehiscence, the ascus wall and its outer layer burst. Thereafter the pleatings of the expansible inner layer are stretched, forming the rather short beak which reaches the hymenial surface. During expansion gliding occurs between the expansible inner layer and an outer part of the endoascus, here described as the ‘inner layer'. In a few sections of aldehyde- fixed material of R. atroflavescens a small laminated plug was observed in the apex of the endoascus. Rhizocarpon-lype asci are considered to be the most archaic in the Lecanorales. This supports a hypothesis that Rhizocarpon is a phylogenetically basal group, linking the evolved Lecanorineae, and possibly also the Peltigerineae and Teloschistineae with not yet recognized bitunicate ancestral forms similar to those occurring in the Patellariacea
