Abstract We investigated whether bacterial motility and chemotaxis in the ocean enables bacteria to approach and follow microscopic, moving, point sources of nutrients. The turbulent nature of the ocean combined with the imprecision of run and tumble chemotaxis has led to the assumption that marine bacteria cannot cluster around microscopic point sources. Recent work, however, shows that marine bacteria use a run and reverse strategy. We examine the ability of marine bacteria that use run and reverse chemotaxis to respond to and follow a moving point source. The addition of the 6 mum in diameter motile algae Pavlova lutheri to cultures of the marine bacteria Pseudoalteromonas haloplanktis and Shewanella putrefaciens revealed bacterial tracking individual free-swimming algae. The marine bacteria travelled at up to 445 mum s(-1) when tracking, up to 237 mum s(-1) when not tracking and up to 126 mum s(-1) in cultures without the algae. Tracking maintained bacteria within one run length of an alga. The bacteria appeared able to steer, consecutively turning up to 12 times toward the motile algae. They recovered from the occasional incorrect turn to continue moving around the swimming alga, indicating that marine bacteria can track moving point sources and form transient phyto-bacterial associations. Our analysis suggests tracking increases nutrient uptake by bringing cells into regions of high nutrient concentrations and by increased advection from high speeds. This result describes what is, apparently, one of the tightest spatial and temporal links between free-living primary and secondary producers in planktonic ecosystems.