Zooxanthellae of the species Gymnodinium (=Symbiodinium) microadriaticum freshly isolated from a variety of hosts (Zoanthus spp., Tridacna crocea, Seriatopora hystrix, Montastrea annularis, Porites furcata, and Stylophora pistillata) in the tropical Pacific Ocean and the Caribbean Sea exhibited saturation uptake kinetics when incubated in seawater enriched with nitrate, nitrite or ammonium. Half-saturation constants (Ks) for nitrate ranged from 0.23 to 3.14 μM, for nitrite from 0.23 to 7.15 μM, and for ammonium from 5.03 to 21.99 μM. Maximum specific uptake rates (Vmax) for ammonium (1.05 to 2.79 d-1) exceeded those for both nitrate (not detectable to 0.54 d-1) and nitrite (not detectable to 0.67 d-1). Light did not affect the uptake rate of any substrate tested, nor did the addition of ammonium in concentrations up to 20 μM reduce the rate of nitrate uptake at Vmax, but nitrite uptake was inhibited by the addition of nitrate to the medium. The inhibition of nitrite uptake by nitrate could not be described either as competitive or non-competitive. Although maximum specific uptake rates obtained for ammonium appear to exceed maximum potential specific growth rate in terms of nitrogen of the algae, the actual growth potential of the algae must be limited not only by nutrient concentrations to which they are exposed, but also by the percentage of assimilated nitrogen that they translocate to their host. Uptake rates obtained for isolated zooxanthellae in this study are similar to those obtained previously for intact coral-zooxanthellae symbioses. This evidence is consistent with the hypothesis that nutrient uptake by intact zooxanthellae-host symbioses occurs by diffusion down a concentration gradient created by localized substrate depletion by the zooxanthellae in host tissue.