The effects of different concentrations of D (-) threo and L (+) threo-chloramphenicol on the growth of Scenedesmus quadricauda have been studied. The results indicate: (a) both isomers caused an increase in the total population largely independent of concentration; (b) at concentrations between 100-200 /[mu]g./ml. both isomers gave a secondary increase in growth after a secondary lag period of 4-6 days; (c) both isomers inhibited the growth rate at concentrations comparable with those which inhibit bacterial growth and protein synthesis. The L (+) threo isomer was twice as effective as the D (-) threo isomer, though it did not give any inhibition below 22 [mu]g./ml. (d) above 50 /[mu]g./ml. The D (-) threo isomer gave a significantly greater lag period than below this concentration. The L (+) threo isomer did not affect the lag period below 25 /[mu]g./ml., but above this concentration, an increase in the concentration lengthened the lag period at the same rate as with the D (-) threo isomer above 50 [mu]/ml.