SUMMARY Washed suspensions of Epidinium spp. grown in vitro and incubated anaero- bically in salts medium engulfed all the bacteria tested at rates ranging from 60 to ~ooo/protozoon/h at a density of 109/ml. The rate of uptake of bacteria was also compared by the volume of medium cleared of bacterialh. On both bases Proteus mirabilis was taken up most rapidly. Some of the bacteria were killed and digested inside the protozoa. The Epidinium spp. took up a wide range of amino acids at rates varying from 6 to 250 nmol/Io6 protozoa/h and incorporated them into protein, mostly without conversion to other amino acids. The protozoa also incorporated purines and pyrimidines into nucleic acid and interconverted adenine and guanine and also uracil and cytosine. Epidinium ecaudatum caudatum took up (14C)glucose more rapidly than Entodinium caudatum and used the glucose carbon for the synthesis of an intracellular glucose polysaccharide and at least eight amino acids, 15 % of the 14C in the protozoa appearing in protein. The protozoa also synthesized protein from starch grains. Evidence has been obtained that Epi. ecaudatum caudatum cannot obtain all the protein it requires for growth from bacteria, free amino acids, glucose or starch and must incorporate protein from the wholemeal flour on which it is grown.