Aerobic anoxygenic phototrophic (AAP) bacteria are bacteriochlorophyll a-containing prokaryotes which can use both light and organic compounds as energy sources. This functional group is ubiquitous in the euphotic zone of the oceans. Nevertheless, life strategies, distribution pat- terns and physiology of AAP bacteria remain largely unknown. We combined infrared fluorometry, microscopic counts and HPLC pigment analysis to characterize free-living and particle-attached AAP bacterial populations. Using a size-fractionation approach, we found that the size distribution of AAP bacteria and the fraction of particle-attached cells varied greatly among different marine environ- ments. In the open sea environments (Atlantic Ocean, offshore Mediterranean Sea), the main portion of AAP bacterial fluorescence was in the <0.8 μm fraction, which indicates that the majority of AAP bacteria in these regions were free-living cells <0.8 μm. In these environments, only a few particle- attached AAP bacteria were found. In coastal Mediterranean waters, the fraction of larger cells increased together with a few particle-attached cells, but >50% of AAP bacteria were free living. In a coastal lagoon and in the deep chlorophyll a maximum at an offshore Mediterranean station, parti- cle-attached AAP bacteria formed up to half of the AAP bacterial community. The results presented here suggest that AAP bacteria can take on either free-living or particle-attached lifestyles depend- ing on environmental conditions.