We present a numerical model of nutrient uptake and photosynthesis during migrations of the marine diatom Rhizosolenia that was developed to estimate fluxes of carbon and nitrogen due to these migrations in the open ocean. The predicted specific rate of increase of Rhizosolenia was 0.11–0.15 day 1 , whereas the total time for one migration cycle ranged between 3 and 5 days. Using published estimates of Rhizosolenia abundance, we estimate that new primary production due to Rhizosolenia migrations ranges between 0.018 and 0.033 mmol N m −2 day −1 . These values represent up to 17% of new production due to turbulent diffusive fluxes of nitrate into the euphotic zone and are of the same order of magnitude as new production due to nitrogen fixation in tropical oceans. Large-scale contributions of Rhizosolenia to oceanic new production are limited by their relatively low standing crop. Variations in the formulation of losses with depth greatly affected gross and net fluxes of carbon and nitrogen. Better characterization of losses of Rhizosolenia and improved estimates of its abundance will help determine more accurately the contributions of Rhizosolenia to global biogeochemical cycles.