Phytochromes are photochromic biliproteins found in plants as well as in some cyanotrophic, photoautotrophic and heterotrophic bacteria. In many bacteria, their function is largely unknown. Here we describe the biochemi-cal and spectroscopic characterization of recombinant bacterial phyto-chrome from the opportunistic pathogen Pseudomonas aeruginosa (PaBphP). The recombinant protein displays all the characteristic features of a bonafide phytochrome.