Aerobic anoxygenic phototrophic (AAnP) bacteria are bacteriochlorophyll a (BChl a)- containing prokaryotes that can use both dissolved organic matter and light as energy sources. AAnP bacteria are widely distributed in aquatic environments where they are expected to play an important role in carbon cycling. However, little is known about their spatio-temporal distribution in marine ecosystems. In this study we examined the dynamics of AAnP bacteria in a coastal saline lagoon from November 2007 to September 2008. AAnP cells were enumerated by infrared (IR) microscopy, and BChl a concentrations were measured by both IR kinetic fluorometry and high-performance liquid chromatography (HPLC). The distribution of AAnP bacteria varied seasonally, but no clear spatial pat- tern emerged. The abundance of these bacteria ranged from 1.0 to 13.5 × 104 cells ml–1 from winter to summer, comprising 0.1 to 3% of total bacterial abundance. Size fractionation of the BChl a fluores- cence signal showed that AAnP bacteria were mainly particle-attached in winter and free-living in spring and summer. BChl a concentrations (up to 108.7 ng l–1), BChl a content per cell (up to 1.7 fg cell–1) and the ratios of BChl a to chlorophyll a (chl a) (up to 15%) were high in spring and summer, suggesting that AAnP bacteria contributed significantly at this time to photosynthetically driven en- ergy production in the lagoon. Temperature and light were the main factors driving seasonal varia- tions in the abundance of AAnP bacteria, while total bacterial abundance was closely related to varia- tions in the concentration of dissolved organic carbon. These results highlight for the first time the numerical importance and the dynamics of AAnP bacteria in a coastal lagoon.