ABSTRACUThe distribution of phosphoribulose kinase (PRK) in the cyanelies of Cyawphora paradoxa Korschikoff and Glucocystis xostochinearwm Itzigsohn was studied by protein A-gold immunoelectron microscopy. In both endocyanomes, antiserum against PRK heavily labeled the thylakoid region of the cyanelles, whereas little or no label was present over the carboxysomes. Antiserum aginst ribulose 1,5-bisphosphate carboxyase/ oxygenase by contrast heavily labeled the carboxysomes of each endocyanome. In vitro studies of PRK distribution in cell-free extacts of C. pardoxa showed that 93% of the enzyme was in the soluble fractin Quantitative immunoelectron mcroscopy showed that more than 99% of the PRK in the cyanelle of C. parwdoxa was lalized in the thylakoid region. We conclude that the carboxysomes of cyanelles like the carboxysomes of autotrophic prokaryotes and the pyrenoids of green algl chloroplasts do not contain phosphoribulose kinase.