Iron oxidizing bacteria are microorganisms difficult to enumerate using conventional bacteriological methods. A modification of the most probable number (MPN) procedure was developed to improve their enumeration. Assays were conducted with Thiobacillus ferrooxidans strains which all showed comparable results. In the dilution range tested, a good correlation was observed between direct microscopic counts and the modified MPN method. The detection limit of the proposed procedure is less than 100 bacterial cells per ml.