To clarify the beneficial roles of naturally occurring organic ligands on the growth of phytoplankton in newly upwelled water, phytoplankton culture experiments using disphotic zone water were conducted to discriminate between the effects of EDTA in the detoxification of certain toxic metal ions and increasing the availability of essential metals. Culture media were prepared by adding EDTA and Chelex-100, separately or in combination, to disphotic zone water samples. Our proposed working hypothesis is that phytoplankton growth can be enhanced by removing toxic metal ions from culture media by Chelex-100 and by detoxification of toxic metal ions or increasing the availability of essential metals by EDTA. A shortening of the lag period and an increase of the specific population growth rate were clearly observed after the addition of Chelex-100; nd EDTA. The effects of EDTA were more considerable than those of Chelex-100; a 17 to 44% in shortening the lag period and a 35 to 56% increase in the growth rate, when comparing the effects of Chelex-100 with those of EDTA. The similar effects of removing toxic metal ion by Chelex-100 as those of detoxification by EDTA suggested that EDTA has a role not only of detoxification but also of increasing the availability of essential metals. The present study suggests that the low productivity in newly upwelled water observed by Barber and Ryther (1969) can be ascribed to both toxic metal ions and a lack of available forms of essential metals because of their low contents of free natural organic ligands.