The biomass of an oxygen-limited autotrophic nitrification/denitrification (OLAND) biofilm reactor was preserved in various ways to find a storage method for both aerobic and anaerobic ammonium-oxidizing bacteria (AerAOB and AnAOB). Storage occurred at -20 degrees C with and without glycerol as cryoprotectant and at 4 and -20 degrees C with and without nitrate as redox buffer. After 2 and 5 months, reactivation of AerAOB and AnAOB was achieved with the biomass stored at 4 degrees C with and without nitrate and at 20 degrees C with nitrate. Moreover, the presence of the AerAOB and AnAOB was confirmed with fluorescent in situ hybridization (FISH). Preservation in a nitrate environment resulted in a lag phase for the AnAOB reactivation. The supplied nitrate was denitrified during storage, and a real-time polymerase chain reaction with nitrifying and denitrifying genes allowed to estimate that at least 1.0 to 6.0% of the OLAND biofilm consisted of denitrifiers. It was concluded that reactivation after longterm storage is possible and that preservation at 4 degrees C without nitrate addition is the recommended storage technique. The possibility to store OLAND biomass will facilitate research on AnAOB and can overcome largerscale start-up and inhibition problems of novel nitrogen processes involving AnAOB.