SUMMARYInability of single cells of coccoid blue‐green algae to form visible colonies on an agar medium has long been a puzzle, especially in view of the high growth rates characterizing coccoid blue‐green algae as a group. WithAnacystis nidulansgrowing on a medium containing citrate as chelating agent, single cells never grew unless catalase was added at time of plating. On a very similar medium but containing EDTA as chelator, single cells ofAnacystisproduced visible colonies.With two other coccoids, both marine isolates, on an EDTA medium (ASP‐2), reasonably quantitative colony counts were obtained. The data are interpreted as a lethal peroxide formation by single cells in light and air by an, as yet, unspecified pathway. Detoxification or reversal of the lethality of small amounts of peroxide, cellularly produced, presumably by the EDTA in the medium, suggests a new role for EDTA in algal media. The data present an insight into the reason (s) for the anaerobic or microaerophilic character of blue–green algae so often observed in