SUMMARYTrebouxiasp., the phycobiont of the lichenRamalina menziesiiTuck., was examined with the electron microscope. Its pyrenoid is characterized structurally as being permeated by interconnected vesiculate membrane systems associated with osmiophilic globules termed pyrenoglobuli. The variety of membrane structure associated with the pyrenoid has been documented with electron micrographs. A model based on serial sections was constructed to show the extent of vesiculation, to demonstrate that the pyrenoglobuli within the pyrenoid matrix are invariably adjacent to the thylakoid membranes within that, matrix, and to emphasize the recurrent thylakoid membrane to pyrenoglobuli relationship. A comparison of the glutaraldehyde‐osmium fixation image of the pyrenoglobuli and pyrenoid matrix with the permanganate fixation image has been included. The structure of the pyrenoid matrix in thin section was examined and appears essentially granula