Short-term experiments were carried out to investigate whether interactions between ammonium (NH4+) and urea uptake regulate the total nitrogen assimilation of the toxic dinoflagellate Alexandrium catenella. To test for strain variability, 5 strains of A. catenella from the NW Mediterranean were used: 3 strains from the Thau lagoon (southern France) and 2 strains from the Catalonia basin (Spain). For each strain, the uptake rate of 1 nutrient (NH4+ or urea) at a reference concentration (10 mu gatN l(-1)) was measured as a function of the increasing concentration of the other nutrient (0 to 10 mu gatN l(-1)). Simultaneous N uptake rates of the distinct nitrogen sources were obtained from N-15-NH4+ and N-15-urea incorporation measurements. A strong inhibition of urea uptake by NH4+ (maximum inhibition, I-max > 55%) was observed exclusively for the French strains. No influence of urea on the NH4+-uptake rate was noted for any strain. Estimation of total N uptake rates revealed that the N-urea uptake inhibition was not a competitive disadvantage for A. catenella cells considering that the reduced N-urea uptake was more than compensated for by NH4+ uptake. Furthermore, the Computation of composite kinetic parameters from total N uptake data suggested that French strains were more competitive than the Spanish ones in an environment characterized by low NH4+ concentrations (<= 5 mu gatN l(-1)) and high Urea concentrations (as 10 mu gatN l(-1)). These N uptake characteristics may reflect particular metabolic adaptations by the strains to their respective environment.