Structures of a series of new antibiotics, agglomerins A, B, C and D, which are active against a variety of anaerobic bacteria, were determined to be 1-acyl-2, 3-dihydroxy-l, 3-butadiene-l-carboxylic acid, (1→3)-γ-lactones, i.e., 2-acyl-4-ylidenetetronic acids with different hydrocarbon chains in the acyl group. Their common chromophore exhibited tautomerism in solution. The relationship of their structure to the activity against anaerobes is discussed.