The photosensitization of microorganisms is potentially useful for sterilization and for the treatment of certain bacterial diseases. Until now, any broad spectrum approach has been inhibited because, although Gram-positive bacteria can be photoinactivated by a range of photosensitizers, Gram-negative bacteria have not usually been susceptible to photosensitized destruction. In the present work, it has been shown that the Gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa, as well as the Gram-positive bacterium Enterococcus seriolicida, can be photoinactivated when illuminated in the presence of a cationic water-soluble zinc pyridinium phthalocyanine (PPC). The degree of photoinactivation is dependent on both the concentration of PPC and the illumination time. In contrast, the three bacteria are not photoinactivated by illumination in the presence of a neutral tetra-diethanolamine phthalocyanine (TDEPC) or negatively charged tetra-sulphonated phthalocyanine (TSPC). Uptake studies have revealed that the lack of activity of TSPC is due to the fact that it has very little affinity for any of the organisms. However, the issue appears to be more complex than simply the gross levels of cellular uptake, since TDEPC and PPC are both taken up by the organisms but only PPC shows activity. This indicates that the localization and subcellular distribution of the phthalocyanines may be a crucial factor in determining their cell killing potential. Further analysis of the uptake data has revealed a cell-bound photosensitizer fraction, which remains tightly associated after several washings, and another weakly bound fraction, which is removed by successive washings. Analysis of the cell killing curves, carried out after successive washings of E. coli exposed to PPC, has revealed that it is the tightly associated fraction that is involved in the photosensitization. Taken together with other data, these results suggest that cationic photosensitizers may have a broader application in the photoinactivation of bacterial cells than the anionic or neutral photosensitizers commonly used in photodynamic therapy.