The structure of 3-carboxy-cis,cis-muconate lactonizing enzyme from Neurospora crassa was determined at 3.0 A resolution. Phase information was derived from a multiwavelength anomalous dispersion (MAD) experiment conducted at three wavelengths using crystals of fully substituted selenomethionine protein. However, the structure determination was not routine owing to the relatively poor quality of the diffraction data and the large number of twofolds in the unit cell. Eventually, 80 selenium sites were identified by the combined use of direct methods and real-space map interpretation. This represents one of the largest selenium substructures solved and used for phasing. Some of the difficulties in the structure determination and the methods used to address them are discussed.