The response of a natural phytoplankton assemblage to different concentrations of Zn 2 + was evaluated by means of a static laboratory bioassay. Aliquots of surface water, taken in autumn from a non-polluted point of the Reconquista River (Buenos Aires, Argentia), were incubated in mineral nutrient media containing 2 5, 10 or 25 mg litre - 1 of Zn 2 + (as zinc chloride). The comparative structure and dynamics of the communities were followed through periodic physico-chemical and biological analyses of samples taken during 24 days of incubation. Under the experimental conditions of the bioassays, the existence of several Zn 2 + tolerant algal species was shown: the most important of them were Chlorella vulgaris Beij. (Chlorophyceae, Chlorococcales), Nitzschia palea(Kutz.) Smith and Gomphonema parvulum (Kutz.) Kutz. (Bacillariophyceae). It was also demonstrated that the algal responses to Zn 2 + were selectively concentration dependent: at 2 5 and 10 mg litre - 1 a stimulatory effect was observed in the diatoms; at 25 mg litre - 1 , diatom toxicity occurred. In contrast, Chlorophyceae growth was stimulated at the maximal Zn level. In general, the diversity, richness and equitability of the community were adversely affected by Zn in a concentration-dependent fashion.