Aims: Investigations were carried out to observe the effect of sunlight on the survival of mesophilic Aeromonas (A. caviae , A. hydrophila and A. sobria ) in high-rate algal pond. Methods and Results: Light damage was estimated by loss of bacterial culturability, using simple beaker experiments. Survival of the strains studied under illuminated conditions was highly affected by the physico-chemical conditions within the microcosms. The inactivation of Aeromonas increased strongly as dissolved oxygen was increased, and was dependent on exogenous sensitizers. This process was enhanced by the high pH of water. Die-off of bacteria was prevented by adding catalase and pyruvate (H-2 O-2 scavengers) into the microcosms. Conclusions: The damage of Aeromonas was mainly due to exogenous photooxidation. Hydrogen peroxide at least was involved in light-induced damage of bacteria. Aeromonas sobria appeared to be slightly less sensitive to photooxidation than A. hydrophila and A. caviae . Significance and Impact of Study: The present study could explain the relative resistance of A. sobria to treatment in wastewater stabilization ponds.