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The cyanobacterial genera Prochlorococcus and Synechococcus are key phototrophic organisms in the open ocean, and ecological interactions between these groups and heterotrophic bacteria have fundamental importance for marine carbon and nutrient cycling. We applied a microfluidic chemotaxis assay to study the chemotactic response of 3 marine bacterial isolates (Pseudoalteromonas haloplanktis, Vibrio alginolyticus and Silicibacter sp. TM1040) to the chemical products of Synechococcus elongatus and Prochlorococcus marinus MED 4Ax. In the ocean, chemical products from these cyanobacteria may be released into the water column via extracellular exudation or cell lysis. Strong and rapid chemotactic responses by all 3 bacterial strains occurred in reaction to the Synechococcus products. P. haloplanktis cells exhibited the strongest attraction, accumulating within the chemoattractant band in …