Phytoplankton species vary widely in their degree of inhibition to low levels of dissolved arsenate (1 – 10 × ambient). Similarly, natural phytoplankton assemblages cultivated outdoors under ambient temperature and nutrient conditions indicate that the observed variance in sensitivity is sufficient to cause a marked change in species composition and succession in dosed assemblages relative to controls, again at very low levels of arsenate. The changes in species that occurred, however, were not always similar to the response predicted from single–species cultures. In addition, a species' level of resistance to arsenate is often not comparable with its resistance to other anthropogenic stress. It is therefore questionable whether predictions concerning environmental consequences of a given pollutant can be made on the basis of laboratory bioassays of a single species.