The three-dimensional structure of L-fuculose-1-phosphate aldolase (FucA) from Escherichia coli was determined by X-ray crystallography at a resolution of 2.13 A. The enzyme is a homotetramer with an M(r) of 23,775 per subunit. Since its activity depends on the presence of metal ions (Zn2+) the enzyme belongs to the class II aldolases. As expected from amino acid sequence comparisons, this first structure of a class II aldolase shows no similarity to the known structures of class I aldolases. It has some unusual features concerning the overall chain fold, the quaternary structure, and the co-ordination of the catalytically active zinc ion. A sequence comparison with the data bank indicated that the middle domain of the enzyme L-ribulose-5-phosphate-4-epimerase is homologous to FucA and may contain an active-center metal ion.