One of the main mechanisms of nanoparticle toxicity is known to be the generation of reactive oxygen species (ROS) which primarily damage cell membranes. However, very limited data on membrane effects in anaerobic environments (where ROS could not be the cause of membrane damage) are available. In the following study, rumen anaerobe Ruminoccocus flavefaciens 007C was used as a bacterial model to assess the potential effects of Al[sub]2O[sub]3 and TiO[sub]2 nanoparticles on embranes in an anaerobic environment. Fatty acid profiles of cultures after exposure to Al[sub]2O[sub]3 or TiO[sub]2 nanoparticles were analyzed and comapred with the profiles of non-exposed cultures or cultures exposed to bulk materials. Analysis revealed dose-effect changes in membrane composition exclusively when cells were exposed to Al[sub]2O[sub]3 nanoparticles in a concentration range of 3-5 g/L, but were not present in cultures exposed to bulk material. On the other hand, the tested concentrations of nano-TiO[sub]2 did not significantly affect the membrane profile of the exposed bacterium. The results suggest the possibility that Al[sub]2O[sub]3 induces changes in bacterial membranes by direct physical interaction, which was supported by TEMimage analysis.