Phytochromes are photochromic photoreceptors with a bilin chromophore that have been found in plants and bacteria. Typical bacterial phytochromes are composed of an N-terminal photosensory chromophore module and a C-terminal protein kinase. The former contains the chromophore, which allows phytochromes to adopt the two interconvertible spectral forms, Pr and Pfr. The N-terminal photosensory module of Agrobacterium phytochrome Agp1, Agp1-M15, was used for crystallization studies. The protein was either assembled with the natural chromophore biliverdin or a sterically locked synthetic biliverdin-derivative, termed 15Za. The last-named adduct does not undergo photoisomerization due to an additional carbon chain between the rings C and D of the chromophore. Both adducts could be crystallized, but the resolution was largely improved by the use of 15Za. Crystals of biliverdin-Agp1-M15 diffract to 6Å resolution and belong to the tetragonal space group I422 with unit cell dimensions a=b=171Å, c=81Å, crystals of 15Za-Agp1-M15 belong to the same space group with similar unit cell dimensions a=b=174Å, c=80Å, but diffract to 3.4Å resolution. Assuming the asymmetric unit to be occupied by one monomer of 55kDa, the unit cell contains 54–55% solvent with a crystal volume per protein mass, V m , of 2.7Å 3 Da −1 .