The selected temperature of Daphnia pulex de Geer as determined in a horizontal still-water temperature gradient showed no effect of acclimation at 15ºC and 20ºC. Exposure to varying concentrations of the DDT-carrier ethanol resulted in decreases in the selected temperatures to values below that shown by untreated daphnia but concentration-dependence was not shown. A trend to a concentration-dependent increase in selected temperature to values higher than that of untreated daphnia was observed in DDT-treated daphnia. DDT increased the precision of selection of temperature as shown by an increase in percent occurrence about the modal temperatures.Untreated and ethanol-treated daphnia showed no behavioural abnormalities in the temperature gradient and low concentrations of DDT apparently had no effect on behaviour. Daphnia exposed to high concentrations of DDT occasionally exhibited "cold and warm shock" responses upon entering these regions of the temperature gradient.It is hypothesized that ethanol lowers and DDT raises the metabolic rate of the whole organism or of its thermal acclimation mechanism, and that these changes in metabolic rate are responsible for the observed changes in selected temperature.