The effect of very low concentrations of DDT on the metabolism of three marine phytoplankton species in batch cultures has been studied. Each species had a different response to exposure. The carbon content was higher under DDT exposure in Dunaliella tertiolecta and Skeletonema costatum and didn't change in Chlamydomonas sp.. The nitrogen metabolism was affected in Chlamydomonas sp. and Skeletonema costatum: the excretion of dissolved organic nitrogen was higher than the controls in both species; assimilation of ammonia was stimulated in Chlamydomonas sp. and the nitrogen content was lower than the controls in Skeletonema costatum. Size and shape of the cells were not affected. Oxygen uptake was stimulated in Chlamydomonas sp.; this is probably a side effect of the increase in cell division rate and the increased ammonia uptake. Dry weight biomass in Chlamydomonas sp. despite the fact that the number of cells was higher. Chlorophyll a and pigment diversity index (D430/D665) were not affected by DDT in the three species. These findings suggest that the effect of DDT contamination in marine phytoplankton communities may occur because of the different kinds of response to DDT exposure in different species.