Pseudomonas putida CP1 is an interesting environmental organism which auto-aggregates when grown under certain conditions. Laboratory studies were carried out to investigate the response of both the free-swimming and the aggregated form of the bacterium to photodisinfection. The response of the planktonic form of the organism was compared with that of E. coli which is widely reported in the literature. A variety of aqueous media were investigated including Milli-Q water, tap water, minimal medium, Phosphate Buffered Saline (PBS) and ¼ Strength Ringers solution. The response of the bacteria to UV-C was carried out in a Heraeus UV-RS3 system (700 ml capacity). Studies using UV-A/B in the presence and absence of 1g/l of TiO2 (titanium dioxide) were carried out in an Aceglass (Vineland, New Jersey USA) reactor vessel model 7841-06 (290 mm and 1l capacity). Photodynamic inactivation was investigated using Rose Bengal and miniaturised LEDs (light-emitting diodes) with Petri dishes. Cell inactivation was determined using both culturable and non-culturable approaches. The response of the free-swimming form of P. putida CP1 to photodisinfection was similar to that for E. coli (DSMZ 498). The composition of the aqueous medium significantly affected the response of the cells to photodisinfection. The aggregated cells were significantly more resistant to photodisinfection than the planktonic cells. Resistance was attributed to the presence of an extracellular matrix comprising carbohydrate, protein and DNA. Photodisinfection using UV-C was the most effective method of disinfection. It was more effective than UV-A/B, in the presence or absence of TiO2, and photodynamic inactivation using Rose Bengal, for the treatment of both planktonic and aggregated cells.