Abstract The presence of azoxyglycosides in the Australian cycad Macrozamia reidlei was examined using high performance liquid chromatography. Cycasin and macrozamia were present in all tissues examined, cycasin being three to 17 times more abundant than macrozamin. The symbiotic organ, the coralloid root, contained 0.16% [g/g (fresh weight)] cycasin and 0.01 % (same unit) macrozamin. Addition of these azoxyglycoside concentrations to nitrogen-fixing Nostoc PCC 73102 cultures, a filamentous heterocystous cyanobacterium originally isolated from Macrozia, inhibited light and dark nitrogenase (acetylene reduction) activity. No effects were observed on in vitro glutamine synthetase activity or net in vivo CO 2 fixation. Cycasin (1.6%) and macrozamin (0.1%), i.e. 10 times the concentrations observed in the coralloid root, decreased phycobiliprotein content by 25 and 45%, 1 and 4 hr after the addition, respectively. The relative distribution of individual phycobiliproteins was not affected.