Zoanthoxanthin, the principal fluorescent pigment of the mediterranean zoanthid Parazoanthus axinellae, has been isolated in crystalline form and identified as 2-amino-3,4-dimethyl-6-dimethylamino-3H-1,3,5,7-tetrazacyclopent[f]azulene 1 from its chemical and spectral properties and by X-ray crystallographic analysis of its 2-chloro-derivative 3. Crystals of 3 are monoclinic, space group P2₁/c, with Z = 4, in a unit cell of dimensions a = 7.282, b = 8.408, c = 25.422 Å, β = 94.31°. The structural analysis, carried out by direct methods and refined to an R value of 0.080, has shown that the molecule is nearly planar, the 7-membered carbocyclic ring assuming a shallow boat conformation while the two imidazole rings are slightly bent in the opposite direction with respect to the folding of the central ring.