The intracellular location of the membrane-bound ammonia monooxygenase (AMO) in all genera of ammonia oxidizing bacteria (Nitrosomonas, Nitrosococcus and Nitrosospira) was determined by electron microscopic immunocytochemistry. Polyclonal antibodies recognizing the two subunits, AmoA- and AmoB-proteins, were used for post-embedding labeling. Ultrathin sections revealed that the AmoB-protein was located in all genera on the cytoplasmic membrane. In cells of Nitrosomonas and Nitrosococus additional but less AmoB-labeling was found on the intracytoplasmic membrane (ICM). In contrast to the detection of AmoB-protein, the AmoA-antibodies failed to detect the AmoA-protein. Based on quantitative immunoblots the extent of ICM in Nitrosomonas eutropha was correlated with the amount of AmoA in the cells. The highest extent of ICM and amount of AmoA was found at low ammonium substrate concentrations.