Many planktonic blue-green algae produce natural chelators which enable them to grow at high pH's in the absence of artificial chelators. The growth of 10 cyanophytes without an added chelator was found to differ widely with the algal species. Bacteria-containing cultures of Anabaena cylindrica, Anacystis nidulans, Lyngbya sp., Microcystis aeruginosa, Nostoc muscorum, and Phormidium foveolarum produced their own chelators and grew just as well as the controls with artificial chelating agents. Bacteria-containing cultures of Anabaena circinalis, Gloeotrichia echinulata, Oscillatoria rubescens, and Aphanizomenon flos-aquae did not produce chelators and, in the absence of artificial agents, grew poorly or perished early. The alga-produced, extracellular chelators were water-soluble and capable of chelating and controlling metal compounds that would exist in colloidal form at pH's above 7. Accordingly, in the absence of artificial chelators, the non-chelator-forming species grew in the filtrates of the chelator-forming algae the same as in the presence of artificial agents. Bacteria were not involved in the formation of natural chelators, since axenic cultures of Anabaena circinalis, Anacystis nidulans, Microcystis aeruginosa, Nostoc muscorum, and Phormidium foveolarum in the absence of artificial chelators performed about the same as the bacteria-associated species. Also, the filtrates of axenic, chelator-forming Anacystis cultures had the same growth-stimulating effect on non-chelator-forming species as filtrates from bacteria-associated cultures. The natural chelators showed partial thermolability.While the growth of chelator-forming species in the absence of artificial chelators was normal during the logarithmic phase, a peculiar, continuing production of total organic matter was observed with strongly declining cell numbers of Lyngbya, Microcystis, and Phormidium. The terminal cultures of these species were gelatinous, owing to the presence of extracellular matter, probably consisting of polysaccharides.