SYNOPSIS. 8‐Hydroxyquinoline (8‐HQ) and cysteine markedly inhibit the synthesis of chlorophylls in non‐proliferating etiolated Euglena gracilis var. bacillaris on illumination. This is thought to be due, at least to some extent, to the binding of Mg2+ ions in the non‐proliferation medium, but largely as a consequence of intracellular chelating action. The effect of 8‐HQ could be reversed by the presence of metal ions higher in the Mellor‐Malley series than Mg2+, e.g., Zn2+ and Co2+. Sodium diethyldithiocarbamate (Na‐DDG) and ethylenediaminetetraacetic acid (EDTA) have no effect on chlorophyll synthesis in non‐proliferation conditions. Growth in the light is more affected than growth in the dark by all these chelating agents, including Na‐DDG and EDTA. This is due to the differential requirement of Mg2+ under these 2 growth conditions.