Availability of toxic trace metals in relation to phytoplankton growth was studied in water of the polluted Westerschelde (southwestern Netherlands). The prominent estuarine diatom Ditylum brightwellii was grown in bioassays with estuarine water (14%o salinity). Selective removal of Cu and Zn, and addition of a chelator (EDTA), had a positive effect on its cell division, whereas addition of Zn had an adverse effect, indicating an influence of in situ levels of Cu or Zn on estuarine phytoplankton growth. To study this in more detail, metal availability was manipulated in order to measure the responses of Ditylum brightwellii at different cellular levels of Cu and Zn. Metal speciation was approached with different metal-adsorbing resins and compared with cellular metal concentrations. Optimum positive correlations between cellular and column-adsorbed fractions of Cu and Zn were obtained, using an aluminum hydroxide-coated sulfonic acid resin (ALSA). Both ALSA-adsorbed and cellular Cu and Zn increased after a 4–h UV irradiation of Westerschelde water, whereas division rates and final cell numbers decreased. Opposite results were obtained after a partial removal of Cu and Zn with Chelex-100®: ALSA-adsorbed and cellular Cu and Zn were relatively lower, and this reduction caused a relative increase of final cell numbers. Addition of Cu caused a relative increase of both ALSA-adsorbed and cellular Zn, probably due to Cu-Zn exchange and a subsequent release of Zn from ligands in Westerschelde water. Increasing amounts of utile Zn or Cu may shorten the exponential-growth phase of Ditylum brightwellii in the Westerschelde. This effect was stronger in a medium from Restronguet Creek (UK) containing micromoles of dissolved Cu and Zn, due to drainage of copper and tin mines.