Photodynamic inactivation (PDI) is currently receiving interest for its potential as an antimicrobial treatment. Although photosensitizing agents and light have been used for medical purposes for a very long time, only a little information is available about the mechanism of PDI for bacteria. Pseudomonas aeruginosa is a Gram negative bacteria involved in chronic infections in cystic fibrosis patients and also one of the commonest agents of hospital acquired infections. In the present study the sensitivity of Pseudomonas aeruginosa to the phototoxic effects of the mono(acridyl)bis(arginyl)porphyrin (MABAP) has been investigated as well as the photophysical and photochemical properties of this cationic porphyrin complexed to [poly(dG-dC)](2) to investigate the mechanisms that lead to bacteria inactivation. Both picosecond time-resolved fluorescence and femtosecond to nanosecond transient absorption measurements give evidence that while MABAP can react through its triplet state and/or an ultrafast electron transfer with guanine, its intercalation between GC base pairs is not the main target of MABAP photoactivity. The analysis of both fluorescence emission and excitation spectra reveals the occurrence of an energy transfer through the DNA double helix between the acridine and porphyrin chromophores of MABAP, as previously observed for the stacked free molecule in solution. This efficient process may lead to the excitation of twice more porphyrin chromophores in MABAP by comparison to other cationic porphyrins.