ABSTRACTMethylammonium, an ammonium analog, was used to study the short‐term kinetics of ammonium uptake in a diatom,Phaeodactylum tricornutumBohlin, and a green alga,Dunaliella tertiolectaButcher. Time courses of methylammonium disappearance were measured over a wide range of initial substrate concentrations for the two species. It was shown that feedback inhibition, described mathematically by a reversible enzyme kinetic model, can be used to explain the data. For the two species, there was good agreement between the kinetic parameters obtained from the analysis of the uptake versus substrate curve and those from the fit of the reversible kinetic model to the time‐course data. All time courses of CH3NH3+disappearance could be described by constants Vmand ks. Ammonium time‐course data show some similarities to its analog, methylammonium. Our study suggests that the apparent change in Vmand kswith time measured after the addition of saturating ammonium concentrations reflects an uncoupling between transport and assimilation of the substrate rather than a real change in the kinetic parameters of the transport mech