We derive a simple prediction about euphotic zone N: P stoichiometry from a large class of models that use saturating nutrient uptake functions to characterize N and P acquisition by phytoplankton. The prediction is: At an ecological steady state, the ratio of phytoplankton N: P to inorganic N: P in the euphotic zone equals the ratio of phytoplankton maximum uptake rates of N and P. We estimate this predicted ratio using nutrient uptake parameters measured in laboratory growth experiments and compare the predicted ratio to empirical observations from long-term sampling in the Atlantic and Pacific oceans. The model predictions for the ratio of phytoplankton N: P to inorganic N: P are at odds with the majority of data from extensive long-term sampling in the Atlantic and the Pacific oceans. This discrepancy calls into question the scope of applicability of ecosystem models that explicitly describe phytoplankton growth as a function of N and P availability. We discuss efforts to resolve this discrepancy, including the need for performing more comprehensive N and P uptake experiments and by reexamining models of nutrient uptake.