MPN, antibody, fluorescent in situ hybridization (FISH) and dot blot hybridization methods for enumeration of ammonia-oxidizing bacteria were applied to various samples from suspended systems. As for FISH, reoptimization of an oligonucleotide probe Nso 190 was attempted. The advantages and disadvantages of these methods were discussed. MPN appeared to have disadvantages of underestimation. Dot blot hybridization had lower detection limit and higher reliability than other methods. It could be applied to mixed liquors and effluents of municipal wastewater treatment plant (WWTPs), lab-scale reactor and enrichment cultures of ammonia-oxidizing bacteria. FISH could be successfully applied to high ammonium loading system such as enrichment cultures, but weak signal, auto-fluorescence and non-specific binding could cause problems when applied to low ammonium loading systems such as effluents of WWTPs and river water. Mixed liquor of municipal WWTP seemed to be a critical case for FISH.