We present four lines of evidence consistent with the suggestion of Gieskes and Kraay that the < 1‐µm particles observed in North Atlantic seawater associated with the carotenoid zeaxanthin are cyanobacteria (Synechococcus spp.). First, four quite different clones of marine Synechococcus contain zeaxanthin as their main carotenoid (50–80% of total carotenoids). Second, none of the other ultraplankters we have yet studied contains more than traces of zeaxanthin, though they contain other pigments potentially useful as chemosystematic markers. Third, none of these other “diagnostic” carotenoids was reported by Gieskes and Kraay. Fourth, the latitudinal gradient of zeaxanthin reported correlates well with our observations of the abundance of Synechococcus, but not of other ultraplankters in the North Atlantic. The circumstantial evidence is strong and merits direct confirmation.

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