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SUMMARY: During utilization of [14C]acetate by non-photosynthetic Euglena gracilis var. bacillaris, 14C was regularly found in trehalose, phosphate esters, glutamate, malate, fumarate, succinate, aspartate, alanine, γ-aminobutyrate, and six unidentified ethanol-soluble compounds. Of the unidentified substances only two, an anthrone-positive material and an ether-soluble substance, were quantitatively important. Small amounts of radioactive laminaribiose, lamin-aritriose, and what were probably higher oligosaccharides of the laminarin series were found by a more sensitive procedure, as well as several additional labelled ninhydrin-positive substances. The kinetics of labelling were consistent with oxidation of acetate via the tricarboxylic acid cycle and assimilation via the glyoxylate cycle and ‘reversed’ glycolysis. There was no indication of any qualitative difference between the intermediates of acetate metabolism and those of oxidation of endogenous reserves. Rapid fluctuations in levels of labelled intermediates were observed, some of which coincided with shoulders or plateaux in the radioactivity of the ethanol-soluble fraction as a whole.