Twenty two diversity, evenness and dominance indices based on phytoplankton community structure were examined for their sensitivity to assess eutrophic conditions. Data from two sampling sites, one eutrophic and one oligotrophic, were used as reference data-sets for evaluating the indices. McNaughton's dominance index, cell number, species number (Hill's N0), Kothe's species deficit, Odum's species per thousand individuals and Evenness E2, E3 seemed to be the most sensitive indices for discriminating between eutrophic and oligotrophic conditions. A case study was also performed using a data-set from ten stations spaced along the coastal area of the city of Rhodes, Aegean Sea, Greece to find out which of the indices were appropriate to assess different levels of eutrophication. Kothe's species deficit, Hill's N0 (species number), Margalef's and Gleason's indices have shown the highest sensitivity to resolve oligotrophic, mesotrophic and eutrophic conditions.