SUMMARYSome metabolic analogs, including azaguanine, azathymine, azauracil, caffeine, 4‐azaleucine,dl‐ethionine, anddl‐p‐fluorophenylanlanine, were examined for their ability to repress the multiplication of algae from natural aquatic sources grown in defined or semidefined media.dl‐ethionine,dl‐p‐fluorophenylalanine, and 4‐azaleucine, in that order, inhibited the multiplication of blue‐green but not other algal groups. The purine and pyrimidine analogs were not inhibitory. In chemically defined axenic media,dl‐ethionine was about 100 times more inhibitory to the blue‐green algaeSynechococcus cedrorumandAnabaena cylindricathan to the eucaryotic algaeOchromonas danicaandEuglena gracilis.The ciliateTetrahymena pyriformis was at least 100‐fold more resistant to ethionine than the algae. The unusual sensitivity of blue‐green algae to ethionine and other amino acid analogs represents an exceptional phylelic character and may be useful in the control of these algae when they become a nuisance. Amino acid analogs such as ethionine may also serve to remove blue‐green algae from cultures one