Upon treatment of glaucocystophytes with penicillin, which interferes with peptidoglycan synthesis, cyanelles stop dividing and lose phycobiliproteins. The symbiophoric vacuole is transformed into a cytolysosome. Gloeochaete and Glaucocystis cells finally die whereas in Cyanophora aposymbiotic flagellates arise which swim actively for several days and divide. It is concluded that the peptidoglycan wall of cyanelles is indispensable for the maintenance of the glaucocystophycean endocytobiosis. Possible consequences of the loss of the peptidoglycan wall during the evolution of chloroplasts are discussed.