Experiments on the availability of iron in culture media have been carried out using seawater from Hood Canal, Washington. When all nutrients (except for Fe‐EDTA) were added and the culture medium was autoclaved, the iron present in the seawater was unavailable for growth of most of the six phytoplankton species studied. EDTA was effective in making such iron available for algal growth in culture media made with freshly collected seawater but not when the water had been stored some days. Both in stored seawater and in autoclaved culture medium, part of the soluble iron was converted into a particulate form.