The research project was initiated to determine the effects of chlorinated hydrocarbons at various concentrations on the growth of euryhaline microalgae, to determine if resistance to DDT occurs in algae with short generation times and with wide tolerances to naturally occurring environmental conditions, and to determine photosynthetic rates for algae treated with chlorinated hydrocarbons. Extensive studies on the effects of DDT on the growth of Platymonas subcordiformis indicate that some degree of resistance occurs naturally in the alga and that depressions in the carrying capacities and in growth rates, especially, in the form of lags, are common at sublethal levels. Growth rates were adversely affected by DDT concentrations at 100 to 200 ppb. At 500 ppb cells never recovered within the experimental period (7 to 10 days). Studies on the effects of DDT at varying salinity levels suggest that cells are less resistant to DDT at higher salinities (22 to 30 PPT).