Chlorophyll-a-specific growth and grazing rates of phytoplankton in three size fractions (0.2 to 2 µm, 2 to 20 µm, and 20 to 153 µm) were determined using dilution assays (n = 38) that were conducted from May to December 2009 in the St. Lawrence River (discharge approx. 7135 m3 s−1) from its headwaters at Lake Ontario to 180 km downstream. There was no discernible difference in phytoplankton growth or grazing rates as a function of river reach sampled. At water temperatures below 20°C specific rates of growth and grazing were closely matched (0.43 day−1), suggesting tight coupling of phytoplankton grazing in the water column. However, specific phytoplankton growth rates exceeded specific grazing rates by 0.29 day−1 when river water temperature exceeded 20°C. Nevertheless, the greater specific growth rate did not manifest in greater phytoplankton biomass with transit downstream suggesting the importance of grazer organisms on benthic surfaces that graze phytoplankton in this reach of a large river. Copyright © 2011 John Wiley & Sons, Ltd.