The structure of a synthetic bile pigment, 3,8,12,17-tetraethyl-4,5-dimethoxy-2,7,13,18-tetramethyl-4,5-dihydrobilin-1,19(21H,24H)-dione, C₃₃H₄₄N₄O₄, has been determined using three-dimensional diffractometer data. It crystallizes in the triclinic space group P with a= 12,755 (2), b= 12.944(s), c= 12.615(2)A, α= 112.27°(2), β= 113.76°(2), γ= 75.45°(2), Z= 2. There is one-half of an n-hexane molecule of crystallization present per formula unit. The structure was solved by direct methods and refined by full-matrix least squares to yield a final R factor of 0.105. The compound, which could be considered as a model for the far-red form (P_fr) of the phytochrome chromphore, belongs to the class of bile pigments known trivially as bilipurpurins. Three of the four pyrrole rings form a conjugated system which is not strictly planar, but is quasi-helical in nature. The fourth ring which is joined to the others by an sp³ carbon atom, is considerably out of the plane of the other three. Bond lengths and angles show a large degree of bond fixation. There is intermolecular hydrogen bonding so that the molecules form hydrogen-bonded dimers.