Brown tide, the uncontrollable bloom of the pelagophycean, Aureococcus anophagefferens, occurred suddenly for the first time in 1985 in bays off Long Island and continues to occur unpredictably. My research focused on the hypothesis that constituents of groundwater influence the initiation of brown tide. Groundwater flows into the bays of Long Island, carrying chemicals that may cause one or a few species of the phytoplankton communities to dominate in local bays. Five species of phytoplankton (Aureococcus anophagefferens, Thalassiosira pseudonanna, Prorocentrum minimum, Synechococcus bacillaris, Nannochloris atomus) were grown in a media containing different concentrations of aldicarb, which is a pesticide commonly used on farms on Long Island. Hypothetically, pesticides, such as aldicarb, may stimulate growth of some species while inhibiting others, depending on concentration. Six flasks of media were made for each species, each with a different concentration of aldicarb (0.1 µg/L, 1 µg/L, 10 µg/L, 100 µg/L, 1000 µg/L, and a control containing no aldicarb.) The flasks were incubated for nineteen days on a 12:12 light-dark cycle. Growth was followed daily using a fluorometer, which measures the amount of chlorophyll in a sample. These data will be used to compare the growth rates of the five species of phytoplankton in order to see which will dominate in the presence of aldicarb. Data from this experiment will be compared to previous experiments with aldicarb breakdown products, aldicarb sulfone and aldicarb sulfoxide, because all three are detected in shallow groundwaters. This research was supported by The Suffolk County Health Services Grant No. 008459.