By developing a method for discriminating between the radioactivities of14C and 65Zn, it has proved possible to measure the influence of zinc upon the carbon fixation rates of natural phytoplankton communities as a function of both the zinc concentrations in the water and the zinc/chlorophyll a ratios in the plant cells. Photosynthesis in the phytoplankton assemblages present at Station L4 in the English Channel during July 1978 was found to be inhibited at zinc concentrations which have been reported to be present in the waters of some other coastal regions around the British Isles. Zinc/chlorophyll a ratios in the phytoplankton were related to the zinc levels in the water by an expression analogous to the Langmuir adsorption isotherm and, presumably due to the near saturation of the zinc binding sites in the plant cells, began to approach their maximal values at zinc concentrations above about 30 μg/l. As the carbon fixation rates were linearly correlated with the zinc/chlorophyll a ratios, these too began to level-off at about the same concentration, higher zinc values causing very little further inhibition. The experimental data are shown to be in good agreement with an expression which describes the effect of zinc upon carbon fixation rates in phytoplankton simply in terms of the zinc binding capacity of the cells and the zinc concentration in the water. This indicates that, as in cultures, the growth rates of natural phytoplankton are also governed by the cellular contents of rate-limiting constituents rather than by the concentrations in the water. © 1979, Marine Biological Association of the United Kingdom. All rights reserved.