Crystals of cyclic green fluorescent protein (cGFP) engineered by the previously reported split intein technology [Iwai et al. (2001), J. Biol. Chem. 276, 16548-16554] were obtained and the structure was solved using molecular replacement. Although the core of the protein can unambiguously be fitted from the first to the last residue of the genuine sequence, the electron density in the region of the linker peptide is rather poor owing to the high water content of the crystals. Therefore, it is concluded that this part of the protein is highly disordered in the present structure and is very flexible. This is supported by the absence of crystal contacts in the linker-peptide region and the fact that the core of the protein exhibits a very similar conformation to that known from other GFP structures, thereby not implicating any constraints arising from the presence of the artificial linker. Nevertheless, the density is consistent with the loop being intact, as confirmed by mass spectroscopy of dissolved crystals. The present structure contains an antiparallel cGFP dimer where the dimer interface is clearly different from other crystal structures featuring two GFP molecules. This adds further support to the fact that the cylinder surface of GFP is rather versatile and can employ various polar and non-polar patches in protein-protein interactions.