Kepone®, the registered trade name for decachlorooctahydro-1,3, 4-metheno-2H-cyclobuta cd pentalen-2-one, inhibited growth and reduced rates of photosynthesis and respiration of Chlorella sorokiniana Shihira and Kraus and Chlorocbccum hypnosporum Starr. The insecticide reduced rates of respiration more than photosynthesis. Uptake of ¹⁴C-Kepone by the algae was proportional to cellular concentration. A net efflux of Kepone was exhibited by Chlorella cells after 30 minutes of incubation, whereas, equilibrium occurred within 15 minutes of incubation for Chlorococcum. Desorption of Kepone was evident when both algal species were removed from Kepone treated solutions and incubated in untreated nutrient solutions.