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The composition, size, and peptide structure of a fatty acid synthetase aggregate from etiolated Euglena gracilis was studied. The fatty acid synthetase was a lipoprotein containing about 40% lipid. Low-angle laser light scattering of the native fatty acid synthetase yielded a molecular weight of 6 · 106 up to concentrations of about 30 μg fatty acid synthetase/ml; at higher concentrations, the molecular weight increased to 11 · 106. Viscometry of the synthetase solutions yielded results that suggested that the asymmetric fatty acid synthetase aggregate formed a 'dimer' at concentrations above 30 μg fatty acid synthetase/ml by side-to-side interaction. The peptide structure of the fatty acid synthetase prepared in the presence of a variety of proteinase inhibitors included at least six peptides of Mr 150000 or less. More than 68% of the protein was in peptides of less than Mr 150000. N-terminal amino acid analysis gave eight different residues all present in integral amounts, seven at about 11% and one at 24% of the total α-N-dansyl amino acids. The Euglena-aggregated fatty acid synthetase appears to be a very large true multienzyme complex.